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bg-background","children":["$","$L3b",null,{"backHref":"/a-level/maths/ccea/questions","basePath":"/a-level/maths/ccea/trigonometry-15/questions","formulaUrl":"https://ccea.org.uk/downloads/docs/Support/General/2019/Mathematical%20Formulae%20and%20Tables.pdf","initialIndex":8,"initialQuestionWindow":[{"id":"327076de-b61b-400a-bde2-937a30472d46","revisionId":"923d49fa-039b-4deb-a28d-e015a3ef854e","questionNumber":null,"questionType":"LA","specification":"","difficulty":2,"difficultyBand":"easy","tier":null,"totalMarks":2,"sourceTitle":null,"calculatorUse":null,"isCalculation":true,"parts":[{"id":"2b72a823-4676-4a0e-933d-bdced993c272","label":"a","content":"Prove the identity:\n\n$$\\frac{1 - 2\\sin^2 x}{\\cos x - \\sin x} \\equiv \\cos x + \\sin x$$","markscheme":"- Recognises $1 - 2\\sin^2 x = \\cos 2x$ and uses $\\cos 2x = \\cos^2 x - \\sin^2 x$ OR substitutes $1 = \\cos^2 x + \\sin^2 x$ M1\n- Simplifies numerator to $\\cos^2 x - \\sin^2 x$, factorises, and cancels to reach RHS A1","marks":2,"order":0,"markingRubric":{"totalMarks":2,"points":[{"id":"p1","marks":1,"description":"Apply the identity $1 - 2\\sin^2 x = \\cos^2 x - \\sin^2 x$ or substitute $1 = \\sin^2 x + \\cos^2 x$.","code":"M"},{"id":"p2","marks":1,"description":"Factorise the resulting numerator as $(\\cos x - \\sin x)(\\cos x + \\sin x)$ and simplify the fraction.","code":"A","acceptable":["LHS = (cos^2 x - sin^2 x) / (cos x - sin x) = cos x + sin x"]}],"finalAnswers":["\\cos x + \\sin x"],"generatedAt":"2026-05-06T14:03:52.906Z","modelVersion":"gemini-3-flash-preview"},"tableImages":[],"explanation":{"methods":[{"id":"pythagorean_substitution","label":"Pythagorean Identity Substitution","steps":[{"type":"text","order":0,"title":"Start with LHS","content":"To prove the identity, we will start with the Left-Hand Side (LHS) of the expression:\n\n$$\\text{LHS} = \\frac{1 - 2\\sin^2 x}{\\cos x - \\sin x}$$"},{"type":"text","order":1,"title":"Substitute the Pythagorean Identity","content":"Recall the fundamental Pythagorean identity:\n\n$$\\cos^2 x + \\sin^2 x = 1$$\n\nWe substitute this expression directly in place of the term $1$ in the numerator of our LHS expression. This step is key to gaining our first mark **(M1)**:\n\n$$\\text{LHS} = \\frac{(\\cos^2 x + \\sin^2 x) - 2\\sin^2 x}{\\cos x - \\sin x}$$"},{"type":"text","order":2,"title":"Simplify the Numerator","content":"Now, we simplify the terms in the numerator by grouping the $\\sin^2 x$ terms:\n\n$$\\text{LHS} = \\frac{\\cos^2 x + \\sin^2 x - 2\\sin^2 x}{\\cos x - \\sin x}$$\n\nSubtracting the terms gives:\n\n$$\\text{LHS} = \\frac{\\cos^2 x - \\sin^2 x}{\\cos x - \\sin x}$$"},{"type":"text","order":3,"title":"Factor and Cancel","content":"We recognize the numerator as a **difference of two squares** of the form $a^2 - b^2 = (a-b)(a+b)$:\n\n$$\\cos^2 x - \\sin^2 x = (\\cos x - \\sin x)(\\cos x + \\sin x)$$\n\nSubstituting this back into our expression, we have:\n\n$$\\text{LHS} = \\frac{(\\cos x - \\sin x)(\\cos x + \\sin x)}{\\cos x - \\sin x}$$\n\nWe can cancel the common factor $(\\cos x - \\sin x)$ in both the numerator and the denominator, leaving us with the Right-Hand Side **(A1)**:\n\n$$\\text{LHS} = \\cos x + \\sin x$$\n\nSince LHS is shown to equal RHS, the proof is complete."}]},{"id":"double_angle","label":"Double-Angle Formulas","steps":[{"type":"text","order":0,"title":"Start with the LHS","content":"To prove the identity, we will start with the Left-Hand Side (LHS) of the expression:\n\n$$\\text{LHS} = \\frac{1 - 2\\sin^2 x}{\\cos x - \\sin x}$$"},{"type":"text","order":1,"title":"Apply double-angle identities","content":"We recognize that the numerator, $1 - 2\\sin^2 x$, is a double-angle formula for $\\cos 2x$:\n\n$$\\cos 2x = 1 - 2\\sin^2 x$$\n\nAnother form of the double-angle formula for $\\cos 2x$ is:\n\n$$\\cos 2x = \\cos^2 x - \\sin^2 x$$\n\nBy equating these, we can rewrite the numerator directly in terms of $\\cos x$ and $\\sin x$:\n\n$$\\text{LHS} = \\frac{\\cos^2 x - \\sin^2 x}{\\cos x - \\sin x}$$"},{"type":"text","order":2,"title":"Factorize the numerator","content":"Next, we recognize the numerator $\\cos^2 x - \\sin^2 x$ as a **difference of two squares** of the form $a^2 - b^2 = (a - b)(a + b)$:\n\n$$\\cos^2 x - \\sin^2 x = (\\cos x - \\sin x)(\\cos x + \\sin x)$$\n\nSubstituting this factorization back into our fraction gives:\n\n$$\\text{LHS} = \\frac{(\\cos x - \\sin x)(\\cos x + \\sin x)}{\\cos x - \\sin x}$$"},{"type":"text","order":3,"title":"Cancel and complete","content":"We can now cancel the common factor of $(\\cos x - \\sin x)$ from both the numerator and the denominator:\n\n$$\\text{LHS} = \\cos x + \\sin x$$\n\nSince the LHS is now equal to the RHS, the proof is complete.\n\n### Markscheme Breakdown\n- **M1**: Awarded for recognizing $1 - 2\\sin^2 x = \\cos 2x$ and converting it to $\\cos^2 x - \\sin^2 x$.\n- **A1**: Awarded for factorizing the numerator, canceling the denominator, and correctly reaching the RHS."}]},{"id":"reverse_multiplication","label":"Working from Right-to-Left","steps":[{"type":"text","order":0,"title":"Understand the approach","content":"To prove the identity using the **Right-to-Left** approach, we want to establish a link between the right-hand side (RHS) and the left-hand side (LHS).\n\nLet's start by multiplying the RHS expression by the denominator of the LHS to see if it simplifies to the numerator of the LHS:\n\n$$\\text{RHS} \\times \\text{Denominator of LHS} = (\\cos x + \\sin x)(\\cos x - \\sin x)$$"},{"type":"text","order":1,"title":"Apply difference of squares","content":"We expand this product using the difference of squares identity, $(a+b)(a-b) = a^2 - b^2$:\n\n$$(\\cos x + \\sin x)(\\cos x - \\sin x) = \\cos^2 x - \\sin^2 x$$"},{"type":"text","order":2,"title":"Use Pythagorean identity","content":"Recall the fundamental Pythagorean identity:\n\n$$\\cos^2 x + \\sin^2 x = 1 \\implies \\cos^2 x = 1 - \\sin^2 x$$\n\nSubstituting this for $\\cos^2 x$ in our expression gives:\n\n$$\\cos^2 x - \\sin^2 x = (1 - \\sin^2 x) - \\sin^2 x$$\n\nSimplifying this expression yields:\n\n$$\\cos^2 x - \\sin^2 x = 1 - 2\\sin^2 x$$\n\nNotice that this is exactly the numerator of our LHS!"},{"type":"text","order":3,"title":"Complete the proof","content":"We have shown that:\n\n$$(\\cos x + \\sin x)(\\cos x - \\sin x) = 1 - 2\\sin^2 x$$\n\nNow, dividing both sides by $(\\cos x - \\sin x)$ isolates the RHS expression:\n\n$$\\cos x + \\sin x = \\frac{1 - 2\\sin^2 x}{\\cos x - \\sin x}$$\n\nThis is exactly LHS = RHS, which completes our proof.\n\n### Markscheme Breakdown\n- **M1**: Awarded for substituting $\\cos^2 x = 1 - \\sin^2 x$ (or equivalent identity work).\n- **A1**: Awarded for successfully completing the simplification to show both sides are equal."}]}],"generatedAt":"2026-06-09T04:15:43.542Z","modelVersion":"gemini-3.5-flash","explanationVersion":"v2"},"options":[]}],"options":[],"explanation":{"methods":[{"id":"2b72a823-4676-4a0e-933d-bdced993c272:pythagorean_substitution","label":"Pythagorean Identity Substitution","steps":[{"type":"text","order":0,"title":"Start with LHS","content":"To prove the identity, we will start with the Left-Hand Side (LHS) of the expression:\n\n$$\\text{LHS} = \\frac{1 - 2\\sin^2 x}{\\cos x - \\sin x}$$"},{"type":"text","order":1,"title":"Substitute the Pythagorean Identity","content":"Recall the fundamental Pythagorean identity:\n\n$$\\cos^2 x + \\sin^2 x = 1$$\n\nWe substitute this expression directly in place of the term $1$ in the numerator of our LHS expression. This step is key to gaining our first mark **(M1)**:\n\n$$\\text{LHS} = \\frac{(\\cos^2 x + \\sin^2 x) - 2\\sin^2 x}{\\cos x - \\sin x}$$"},{"type":"text","order":2,"title":"Simplify the Numerator","content":"Now, we simplify the terms in the numerator by grouping the $\\sin^2 x$ terms:\n\n$$\\text{LHS} = \\frac{\\cos^2 x + \\sin^2 x - 2\\sin^2 x}{\\cos x - \\sin x}$$\n\nSubtracting the terms gives:\n\n$$\\text{LHS} = \\frac{\\cos^2 x - \\sin^2 x}{\\cos x - \\sin x}$$"},{"type":"text","order":3,"title":"Factor and Cancel","content":"We recognize the numerator as a **difference of two squares** of the form $a^2 - b^2 = (a-b)(a+b)$:\n\n$$\\cos^2 x - \\sin^2 x = (\\cos x - \\sin x)(\\cos x + \\sin x)$$\n\nSubstituting this back into our expression, we have:\n\n$$\\text{LHS} = \\frac{(\\cos x - \\sin x)(\\cos x + \\sin x)}{\\cos x - \\sin x}$$\n\nWe can cancel the common factor $(\\cos x - \\sin x)$ in both the numerator and the denominator, leaving us with the Right-Hand Side **(A1)**:\n\n$$\\text{LHS} = \\cos x + \\sin x$$\n\nSince LHS is shown to equal RHS, the proof is complete."}]},{"id":"2b72a823-4676-4a0e-933d-bdced993c272:double_angle","label":"Double-Angle Formulas","steps":[{"type":"text","order":0,"title":"Start with the LHS","content":"To prove the identity, we will start with the Left-Hand Side (LHS) of the expression:\n\n$$\\text{LHS} = \\frac{1 - 2\\sin^2 x}{\\cos x - \\sin x}$$"},{"type":"text","order":1,"title":"Apply double-angle identities","content":"We recognize that the numerator, $1 - 2\\sin^2 x$, is a double-angle formula for $\\cos 2x$:\n\n$$\\cos 2x = 1 - 2\\sin^2 x$$\n\nAnother form of the double-angle formula for $\\cos 2x$ is:\n\n$$\\cos 2x = \\cos^2 x - \\sin^2 x$$\n\nBy equating these, we can rewrite the numerator directly in terms of $\\cos x$ and $\\sin x$:\n\n$$\\text{LHS} = \\frac{\\cos^2 x - \\sin^2 x}{\\cos x - \\sin x}$$"},{"type":"text","order":2,"title":"Factorize the numerator","content":"Next, we recognize the numerator $\\cos^2 x - \\sin^2 x$ as a **difference of two squares** of the form $a^2 - b^2 = (a - b)(a + b)$:\n\n$$\\cos^2 x - \\sin^2 x = (\\cos x - \\sin x)(\\cos x + \\sin x)$$\n\nSubstituting this factorization back into our fraction gives:\n\n$$\\text{LHS} = \\frac{(\\cos x - \\sin x)(\\cos x + \\sin x)}{\\cos x - \\sin x}$$"},{"type":"text","order":3,"title":"Cancel and complete","content":"We can now cancel the common factor of $(\\cos x - \\sin x)$ from both the numerator and the denominator:\n\n$$\\text{LHS} = \\cos x + \\sin x$$\n\nSince the LHS is now equal to the RHS, the proof is complete.\n\n### Markscheme Breakdown\n- **M1**: Awarded for recognizing $1 - 2\\sin^2 x = \\cos 2x$ and converting it to $\\cos^2 x - \\sin^2 x$.\n- **A1**: Awarded for factorizing the numerator, canceling the denominator, and correctly reaching the RHS."}]},{"id":"2b72a823-4676-4a0e-933d-bdced993c272:reverse_multiplication","label":"Working from Right-to-Left","steps":[{"type":"text","order":0,"title":"Understand the approach","content":"To prove the identity using the **Right-to-Left** approach, we want to establish a link between the right-hand side (RHS) and the left-hand side (LHS).\n\nLet's start by multiplying the RHS expression by the denominator of the LHS to see if it simplifies to the numerator of the LHS:\n\n$$\\text{RHS} \\times \\text{Denominator of LHS} = (\\cos x + \\sin x)(\\cos x - \\sin x)$$"},{"type":"text","order":1,"title":"Apply difference of squares","content":"We expand this product using the difference of squares identity, $(a+b)(a-b) = a^2 - b^2$:\n\n$$(\\cos x + \\sin x)(\\cos x - \\sin x) = \\cos^2 x - \\sin^2 x$$"},{"type":"text","order":2,"title":"Use Pythagorean identity","content":"Recall the fundamental Pythagorean identity:\n\n$$\\cos^2 x + \\sin^2 x = 1 \\implies \\cos^2 x = 1 - \\sin^2 x$$\n\nSubstituting this for $\\cos^2 x$ in our expression gives:\n\n$$\\cos^2 x - \\sin^2 x = (1 - \\sin^2 x) - \\sin^2 x$$\n\nSimplifying this expression yields:\n\n$$\\cos^2 x - \\sin^2 x = 1 - 2\\sin^2 x$$\n\nNotice that this is exactly the numerator of our LHS!"},{"type":"text","order":3,"title":"Complete the proof","content":"We have shown that:\n\n$$(\\cos x + \\sin x)(\\cos x - \\sin x) = 1 - 2\\sin^2 x$$\n\nNow, dividing both sides by $(\\cos x - \\sin x)$ isolates the RHS expression:\n\n$$\\cos x + \\sin x = \\frac{1 - 2\\sin^2 x}{\\cos x - \\sin x}$$\n\nThis is exactly LHS = RHS, which completes our proof.\n\n### Markscheme Breakdown\n- **M1**: Awarded for substituting $\\cos^2 x = 1 - \\sin^2 x$ (or equivalent identity work).\n- **A1**: Awarded for successfully completing the simplification to show both sides are equal."}]}],"generatedAt":"2026-06-09T04:15:56.845Z","modelVersion":"gemini-3.5-flash","explanationVersion":"v2"},"firstAttemptFullMarks":null,"walkthroughVideo":null},{"id":"934c88ce-aae4-46e7-8f4e-4af676f41791","revisionId":"a55d597d-e871-4976-b388-129be38b081a","questionNumber":null,"questionType":"LA","specification":"","difficulty":2,"difficultyBand":"easy","tier":null,"totalMarks":2,"sourceTitle":null,"calculatorUse":null,"isCalculation":true,"parts":[{"id":"4f2efc02-2b6f-4298-af8b-24851da426b5","label":"a","content":"Prove the identity:\n\n$$\\frac{\\cos 4A}{\\cos 2A + \\sin 2A} \\equiv \\cos 2A - \\sin 2A$$","markscheme":"- Substitutes $\\cos 4A = \\cos^2 2A - \\sin^2 2A$ M1\n- Factorises numerator as $(\\cos 2A - \\sin 2A)(\\cos 2A + \\sin 2A)$ and cancels $(\\cos 2A + \\sin 2A)$ to reach RHS A1","marks":2,"order":0,"markingRubric":{"totalMarks":2,"points":[{"id":"p1","marks":1,"description":"Apply the double angle identity to cos(4A) to get $\\cos^2 2A - \\sin^2 2A$.","code":"M"},{"id":"p2","marks":1,"description":"Correctly factorise and cancel the denominator terms to obtain the RHS.","code":"A","acceptable":["LHS = (cos 2A - sin 2A)(cos 2A + sin 2A) / (cos 2A + sin 2A) = cos 2A - sin 2A"]}],"finalAnswers":["\\cos 2A - \\sin 2A"],"generatedAt":"2026-05-06T14:03:54.761Z","modelVersion":"gemini-3-flash-preview"},"tableImages":[],"explanation":{"methods":[{"id":"lhs-to-rhs","label":"LHS to RHS Expansion","steps":[{"type":"text","order":0,"title":"Start with LHS","content":"To prove the identity, we will start with the Left-Hand Side (LHS) of the expression and simplify it step-by-step to reach the Right-Hand Side (RHS):\n\n$$\\text{LHS} = \\frac{\\cos 4A}{\\cos 2A + \\sin 2A}$$,"},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$\\cos(2\\theta) = \\cos^2\\theta - \\sin^2\\theta$"},{"latex":"$$\\cos(\\htmlData{anchor=angle-double}{4A}) = \\cos^2(\\htmlData{anchor=angle-half-1}{2A}) - \\sin^2(\\htmlData{anchor=angle-half-2}{2A})$"},{"latex":"$$\\text{LHS} = \\frac{\\htmlData{anchor=num-identity}{\\cos^2 2A - \\sin^2 2A}}{\\cos 2A + \\sin 2A}$"}],"highlights":[{"color":"sky","style":"box","anchor":"num-identity"}],"connections":[]},"order":1,"title":"Apply double-angle identity","description":"Recall the double-angle identity for cosine: $$\\cos(2\\theta) = \\cos^2\\theta - \\sin^2\\theta$$ Let's apply this identity by substituting $\\theta = 2A$ to rewrite the numerator $\\cos 4A$."},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$x^2 - y^2 = (x - y)(x + y)$"},{"latex":"$$\\text{LHS} = \\frac{\\htmlData{anchor=factored-num}{\\color{@purple}{(\\cos 2A - \\sin 2A)(\\cos 2A + \\sin 2A)}}}{\\htmlData{anchor=denom}{\\cos 2A + \\sin 2A}}$"}],"highlights":[{"color":"purple","style":"underline","anchor":"factored-num"}],"connections":[]},"order":2,"title":"Factorise the numerator","description":"The numerator $\\cos^2 2A - \\sin^2 2A$ is a difference of two squares. We can factorise it using the algebraic pattern $x^2 - y^2 = (x - y)(x + y)$."},{"type":"text","order":3,"title":"Cancel common factors","content":"Now, we can cancel the common factor of $(\\cos 2A + \\sin 2A)$ from both the numerator and the denominator:\n\n$$\\text{LHS} = \\frac{(\\cos 2A - \\sin 2A)\\cancel{(\\cos 2A + \\sin 2A)}}{\\cancel{\\cos 2A + \\sin 2A}}$$\n\nThis leaves us with:\n\n$$\\text{LHS} = \\cos 2A - \\sin 2A = \\text{RHS}$$\n\nThis completes the proof. (This satisfies the **M1** mark for substituting the double-angle identity and the **A1** mark for factorising and cancelling to reach the final simplified expression)."}]},{"id":"rhs-to-lhs","label":"RHS to LHS Manipulation","steps":[{"type":"text","order":0,"title":"Start with the RHS","content":"To prove the identity using RHS to LHS manipulation, we start with the Right-Hand Side (RHS):\n\n$$\\text{RHS} = \\cos 2A - \\sin 2A$$"},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$\\text{RHS} = \\htmlData{anchor=rhs-expr}{\\color{@sky}{\\cos 2A - \\sin 2A}}$"},{"latex":"$$\\text{RHS} = \\frac{(\\htmlData{anchor=rhs-sub}{\\color{@sky}{\\cos 2A - \\sin 2A}})(\\htmlData{anchor=conj-term}{\\color{@amber}{\\cos 2A + \\sin 2A}})}{\\htmlData{anchor=denom-term}{\\color{@amber}{\\cos 2A + \\sin 2A}}}$"}],"highlights":[{"color":"sky","style":"text-color","anchor":"rhs-expr"},{"color":"sky","style":"text-color","anchor":"rhs-sub"},{"color":"amber","style":"text-color","anchor":"conj-term"},{"color":"amber","style":"text-color","anchor":"denom-term"}],"connections":[{"to":"rhs-sub","from":"rhs-expr","color":"sky","label":"RHS","style":"solid"}]},"order":1,"title":"Multiply and divide","description":"To introduce the fractional structure of the LHS, we multiply and divide the RHS expression by $(\\cos 2A + \\sin 2A)$."},{"type":"text","order":2,"title":"Expand the numerator","content":"The numerator is now in the form of a difference of two squares:\n\n$$(x - y)(x + y) = x^2 - y^2$$\n\nSubstituting $x = \\cos 2A$ and $y = \\sin 2A$, we expand the numerator:\n\n$$\\text{RHS} = \\frac{\\cos^2 2A - \\sin^2 2A}{\\cos 2A + \\sin 2A}$$"},{"type":"text","order":3,"title":"Apply double-angle identity","content":"Recall the double-angle identity for cosine:\n\n$$\\cos 2\\theta = \\cos^2 \\theta - \\sin^2 \\theta$$\n\nLetting $\\theta = 2A$, we get:\n\n$$\\cos 4A = \\cos^2 2A - \\sin^2 2A$$\n\nSubstituting this back into the numerator of our expression yields:\n\n$$\\text{RHS} = \\frac{\\cos 4A}{\\cos 2A + \\sin 2A}$$\n\nThis is identical to the Left-Hand Side (LHS), completing our proof:\n\n$$\\text{RHS} \\equiv \\text{LHS}$$"}]}],"generatedAt":"2026-06-09T07:41:10.830Z","modelVersion":"gemini-3.5-flash","explanationVersion":"v2"},"options":[]}],"options":[],"explanation":{"methods":[{"id":"4f2efc02-2b6f-4298-af8b-24851da426b5:lhs-to-rhs","label":"LHS to RHS Expansion","steps":[{"type":"text","order":0,"title":"Start with LHS","content":"To prove the identity, we will start with the Left-Hand Side (LHS) of the expression and simplify it step-by-step to reach the Right-Hand Side (RHS):\n\n$$\\text{LHS} = \\frac{\\cos 4A}{\\cos 2A + \\sin 2A}$$,"},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$\\cos(2\\theta) = \\cos^2\\theta - \\sin^2\\theta$"},{"latex":"$$\\cos(\\htmlData{anchor=angle-double}{4A}) = \\cos^2(\\htmlData{anchor=angle-half-1}{2A}) - \\sin^2(\\htmlData{anchor=angle-half-2}{2A})$"},{"latex":"$$\\text{LHS} = \\frac{\\htmlData{anchor=num-identity}{\\cos^2 2A - \\sin^2 2A}}{\\cos 2A + \\sin 2A}$"}],"highlights":[{"color":"sky","style":"box","anchor":"num-identity"}],"connections":[]},"order":1,"title":"Apply double-angle identity","description":"Recall the double-angle identity for cosine: $$\\cos(2\\theta) = \\cos^2\\theta - \\sin^2\\theta$$ Let's apply this identity by substituting $\\theta = 2A$ to rewrite the numerator $\\cos 4A$."},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$x^2 - y^2 = (x - y)(x + y)$"},{"latex":"$$\\text{LHS} = \\frac{\\htmlData{anchor=factored-num}{\\color{@purple}{(\\cos 2A - \\sin 2A)(\\cos 2A + \\sin 2A)}}}{\\htmlData{anchor=denom}{\\cos 2A + \\sin 2A}}$"}],"highlights":[{"color":"purple","style":"underline","anchor":"factored-num"}],"connections":[]},"order":2,"title":"Factorise the numerator","description":"The numerator $\\cos^2 2A - \\sin^2 2A$ is a difference of two squares. We can factorise it using the algebraic pattern $x^2 - y^2 = (x - y)(x + y)$."},{"type":"text","order":3,"title":"Cancel common factors","content":"Now, we can cancel the common factor of $(\\cos 2A + \\sin 2A)$ from both the numerator and the denominator:\n\n$$\\text{LHS} = \\frac{(\\cos 2A - \\sin 2A)\\cancel{(\\cos 2A + \\sin 2A)}}{\\cancel{\\cos 2A + \\sin 2A}}$$\n\nThis leaves us with:\n\n$$\\text{LHS} = \\cos 2A - \\sin 2A = \\text{RHS}$$\n\nThis completes the proof. (This satisfies the **M1** mark for substituting the double-angle identity and the **A1** mark for factorising and cancelling to reach the final simplified expression)."}]},{"id":"4f2efc02-2b6f-4298-af8b-24851da426b5:rhs-to-lhs","label":"RHS to LHS Manipulation","steps":[{"type":"text","order":0,"title":"Start with the RHS","content":"To prove the identity using RHS to LHS manipulation, we start with the Right-Hand Side (RHS):\n\n$$\\text{RHS} = \\cos 2A - \\sin 2A$$"},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$\\text{RHS} = \\htmlData{anchor=rhs-expr}{\\color{@sky}{\\cos 2A - \\sin 2A}}$"},{"latex":"$$\\text{RHS} = \\frac{(\\htmlData{anchor=rhs-sub}{\\color{@sky}{\\cos 2A - \\sin 2A}})(\\htmlData{anchor=conj-term}{\\color{@amber}{\\cos 2A + \\sin 2A}})}{\\htmlData{anchor=denom-term}{\\color{@amber}{\\cos 2A + \\sin 2A}}}$"}],"highlights":[{"color":"sky","style":"text-color","anchor":"rhs-expr"},{"color":"sky","style":"text-color","anchor":"rhs-sub"},{"color":"amber","style":"text-color","anchor":"conj-term"},{"color":"amber","style":"text-color","anchor":"denom-term"}],"connections":[{"to":"rhs-sub","from":"rhs-expr","color":"sky","label":"RHS","style":"solid"}]},"order":1,"title":"Multiply and divide","description":"To introduce the fractional structure of the LHS, we multiply and divide the RHS expression by $(\\cos 2A + \\sin 2A)$."},{"type":"text","order":2,"title":"Expand the numerator","content":"The numerator is now in the form of a difference of two squares:\n\n$$(x - y)(x + y) = x^2 - y^2$$\n\nSubstituting $x = \\cos 2A$ and $y = \\sin 2A$, we expand the numerator:\n\n$$\\text{RHS} = \\frac{\\cos^2 2A - \\sin^2 2A}{\\cos 2A + \\sin 2A}$$"},{"type":"text","order":3,"title":"Apply double-angle identity","content":"Recall the double-angle identity for cosine:\n\n$$\\cos 2\\theta = \\cos^2 \\theta - \\sin^2 \\theta$$\n\nLetting $\\theta = 2A$, we get:\n\n$$\\cos 4A = \\cos^2 2A - \\sin^2 2A$$\n\nSubstituting this back into the numerator of our expression yields:\n\n$$\\text{RHS} = \\frac{\\cos 4A}{\\cos 2A + \\sin 2A}$$\n\nThis is identical to the Left-Hand Side (LHS), completing our proof:\n\n$$\\text{RHS} \\equiv \\text{LHS}$$"}]}],"generatedAt":"2026-06-09T07:41:22.307Z","modelVersion":"gemini-3.5-flash","explanationVersion":"v2"},"firstAttemptFullMarks":null,"walkthroughVideo":null},{"id":"cd1dffd5-3636-4a2e-a756-86ef66e4090d","revisionId":"3c5203f9-3ac9-429a-b7b5-69404536e42d","questionNumber":"6","questionType":"LA","specification":"","difficulty":2,"difficultyBand":"easy","tier":null,"totalMarks":2,"sourceTitle":"7.1 Addition Formulae","calculatorUse":null,"isCalculation":true,"parts":[{"id":"2df7cc75-cbbc-423c-95f4-92789ff477ef","label":"0","content":"Prove the identity:\n\n$$\\frac{\\cos 2x}{\\cos x + \\sin x} \\equiv \\cos x - \\sin x$$","markscheme":"- Substitutes $\\cos 2x = \\cos^2 x - \\sin^2 x$ M1\n- Factorises numerator as $(\\cos x - \\sin x)(\\cos x + \\sin x)$ and cancels denominator to reach RHS A1","marks":2,"order":0,"markingRubric":{"totalMarks":2,"points":[{"id":"p1","marks":1,"description":"Uses the identity $\\cos 2x = \\cos^2 x - \\sin^2 x$.","code":"M"},{"id":"p2","marks":1,"description":"Factorises the difference of two squares and cancels the common factor.","code":"A"}],"finalAnswers":[],"generatedAt":"2026-05-06T03:35:10.313Z","modelVersion":"gemini-3-flash-preview"},"tableImages":[],"explanation":{"methods":[{"id":"lhs_to_rhs","label":"Simplifying LHS to RHS","steps":[{"type":"text","order":0,"title":"Identify the LHS","content":"To prove the identity using the LHS to RHS method, we begin by writing down the expression on the Left-Hand Side (LHS):\n\n$$\\text{LHS} = \\frac{\\cos 2x}{\\cos x + \\sin x}$$"},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$\\text{Recall the double-angle identity: } \\htmlData{anchor=identity}{\\cos 2x} = \\htmlData{anchor=expanded}{\\cos^2 x - \\sin^2 x}$"},{"latex":"$$\\text{LHS} = \\frac{\\htmlData{anchor=substitute}{\\cos^2 x - \\sin^2 x}}{\\cos x + \\sin x}$"}],"highlights":[{"color":"sky","style":"box","anchor":"identity"},{"color":"sky","style":"text-color","anchor":"expanded"},{"color":"sky","style":"text-color","anchor":"substitute"}],"connections":[{"to":"substitute","from":"expanded","color":"sky","label":"substitute","style":"solid"}]},"order":1,"title":"Apply double-angle identity","description":"We substitute the double-angle identity for cosine, $\\cos 2x = \\cos^2 x - \\sin^2 x$, into the numerator. This step earns the first mark."},{"type":"text","order":2,"title":"Factorise the numerator","content":"Now notice that the numerator $\\cos^2 x - \\sin^2 x$ is a difference of two squares in the form $a^2 - b^2 = (a - b)(a + b)$.\n\nFactoring the numerator gives us:\n\n$$\\text{LHS} = \\frac{(\\cos x - \\sin x)(\\cos x + \\sin x)}{\\cos x + \\sin x}$$"},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$\\text{LHS} = \\frac{(\\cos x - \\sin x)\\htmlData{anchor=num-factor}{\\color{@pink}{(\\cos x + \\sin x)}}}{\\htmlData{anchor=den-factor}{\\color{@pink}{\\cos x + \\sin x}}}$"},{"latex":"$$\\text{LHS} = \\htmlData{anchor=rhs}{\\cos x - \\sin x}$"}],"highlights":[{"color":"pink","style":"underline","anchor":"num-factor"},{"color":"pink","style":"underline","anchor":"den-factor"}],"connections":[{"to":"rhs","from":"num-factor","color":"pink","label":"cancel","style":"dashed"}]},"order":3,"title":"Simplify and reach RHS","description":"We can cancel the common factor of $(\\cos x + \\sin x)$ in the numerator and denominator to simplify the expression, completing our proof."},{"type":"text","order":4,"title":"State the final conclusion","content":"Since the simplified LHS is identical to the RHS, we have proven the identity:\n\n$$\\frac{\\cos 2x}{\\cos x + \\sin x} \\equiv \\cos x - \\sin x$$"}]},{"id":"rhs_to_lhs","label":"Expanding RHS to LHS","steps":[{"type":"text","order":0,"title":"Start with RHS","content":"To prove the identity using this method, we begin with the right-hand side (RHS) of the equation:\n\n$$\\text{RHS} = \\cos x - \\sin x$$"},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$\\htmlData{anchor=rhs}{\\cos x - \\sin x}$"},{"latex":"$$= \\frac{(\\htmlData{anchor=rhs-sub}{\\cos x - \\sin x})(\\htmlData{anchor=mult}{\\cos x + \\sin x})}{\\htmlData{anchor=div}{\\cos x + \\sin x}}$"}],"highlights":[{"color":"sky","style":"text-color","anchor":"rhs"},{"color":"amber","style":"box","anchor":"mult"},{"color":"amber","style":"box","anchor":"div"}],"connections":[{"to":"rhs-sub","from":"rhs","color":"sky","label":null,"style":"solid"}]},"order":1,"title":"Multiply and divide","description":"We multiply and divide our RHS expression by the conjugate term $(\\cos x + \\sin x)$. This does not change the value of the expression, as it is equivalent to multiplying by $1$."},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$= \\frac{\\htmlData{anchor=num-fact}{(\\cos x - \\sin x)(\\cos x + \\sin x)}}{\\cos x + \\sin x}$"},{"latex":"$$= \\frac{\\htmlData{anchor=num-exp}{\\cos^2 x - \\sin^2 x}}{\\cos x + \\sin x}$"}],"highlights":[{"color":"purple","style":"text-color","anchor":"num-fact"},{"color":"purple","style":"text-color","anchor":"num-exp"}],"connections":[{"to":"num-exp","from":"num-fact","color":"purple","label":"expand","style":"solid"}]},"order":2,"title":"Expand the numerator","description":"Next, simplify the numerator by expanding the product. Notice that this is a difference of two squares in the form $(a - b)(a + b) = a^2 - b^2$, where $a = \\cos x$ and $b = \\sin x$."},{"type":"text","order":3,"title":"Apply double-angle identity","content":"Now we recall the double-angle formula for cosine from our trigonometric identities:\n\n$$\\cos 2x \\equiv \\cos^2 x - \\sin^2 x$$\n\nSubstituting this into the numerator gives us:\n\n$$\\frac{\\cos 2x}{\\cos x + \\sin x}$$"},{"type":"text","order":4,"title":"State final conclusion","content":"This matches the left-hand side (LHS) of our original identity:\n\n$$\\text{RHS} \\equiv \\text{LHS}$$\n\nThus, we have successfully proven that:\n\n$$\\frac{\\cos 2x}{\\cos x + \\sin x} \\equiv \\cos x - \\sin x$$"}]}],"generatedAt":"2026-06-09T09:22:44.940Z","modelVersion":"gemini-3.5-flash","explanationVersion":"v2"},"options":[]}],"options":[],"explanation":{"methods":[{"id":"2df7cc75-cbbc-423c-95f4-92789ff477ef:lhs_to_rhs","label":"Simplifying LHS to RHS","steps":[{"type":"text","order":0,"title":"Identify the LHS","content":"To prove the identity using the LHS to RHS method, we begin by writing down the expression on the Left-Hand Side (LHS):\n\n$$\\text{LHS} = \\frac{\\cos 2x}{\\cos x + \\sin x}$$"},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$\\text{Recall the double-angle identity: } \\htmlData{anchor=identity}{\\cos 2x} = \\htmlData{anchor=expanded}{\\cos^2 x - \\sin^2 x}$"},{"latex":"$$\\text{LHS} = \\frac{\\htmlData{anchor=substitute}{\\cos^2 x - \\sin^2 x}}{\\cos x + \\sin x}$"}],"highlights":[{"color":"sky","style":"box","anchor":"identity"},{"color":"sky","style":"text-color","anchor":"expanded"},{"color":"sky","style":"text-color","anchor":"substitute"}],"connections":[{"to":"substitute","from":"expanded","color":"sky","label":"substitute","style":"solid"}]},"order":1,"title":"Apply double-angle identity","description":"We substitute the double-angle identity for cosine, $\\cos 2x = \\cos^2 x - \\sin^2 x$, into the numerator. This step earns the first mark."},{"type":"text","order":2,"title":"Factorise the numerator","content":"Now notice that the numerator $\\cos^2 x - \\sin^2 x$ is a difference of two squares in the form $a^2 - b^2 = (a - b)(a + b)$.\n\nFactoring the numerator gives us:\n\n$$\\text{LHS} = \\frac{(\\cos x - \\sin x)(\\cos x + \\sin x)}{\\cos x + \\sin x}$$"},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$\\text{LHS} = \\frac{(\\cos x - \\sin x)\\htmlData{anchor=num-factor}{\\color{@pink}{(\\cos x + \\sin x)}}}{\\htmlData{anchor=den-factor}{\\color{@pink}{\\cos x + \\sin x}}}$"},{"latex":"$$\\text{LHS} = \\htmlData{anchor=rhs}{\\cos x - \\sin x}$"}],"highlights":[{"color":"pink","style":"underline","anchor":"num-factor"},{"color":"pink","style":"underline","anchor":"den-factor"}],"connections":[{"to":"rhs","from":"num-factor","color":"pink","label":"cancel","style":"dashed"}]},"order":3,"title":"Simplify and reach RHS","description":"We can cancel the common factor of $(\\cos x + \\sin x)$ in the numerator and denominator to simplify the expression, completing our proof."},{"type":"text","order":4,"title":"State the final conclusion","content":"Since the simplified LHS is identical to the RHS, we have proven the identity:\n\n$$\\frac{\\cos 2x}{\\cos x + \\sin x} \\equiv \\cos x - \\sin x$$"}]},{"id":"2df7cc75-cbbc-423c-95f4-92789ff477ef:rhs_to_lhs","label":"Expanding RHS to LHS","steps":[{"type":"text","order":0,"title":"Start with RHS","content":"To prove the identity using this method, we begin with the right-hand side (RHS) of the equation:\n\n$$\\text{RHS} = \\cos x - \\sin x$$"},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$\\htmlData{anchor=rhs}{\\cos x - \\sin x}$"},{"latex":"$$= \\frac{(\\htmlData{anchor=rhs-sub}{\\cos x - \\sin x})(\\htmlData{anchor=mult}{\\cos x + \\sin x})}{\\htmlData{anchor=\\div}{\\cos x + \\sin x}}$"}],"highlights":[{"color":"sky","style":"text-color","anchor":"rhs"},{"color":"amber","style":"box","anchor":"mult"},{"color":"amber","style":"box","anchor":"div"}],"connections":[{"to":"rhs-sub","from":"rhs","color":"sky","label":null,"style":"solid"}]},"order":1,"title":"Multiply and divide","description":"We multiply and divide our RHS expression by the conjugate term $(\\cos x + \\sin x)$. This does not change the value of the expression, as it is equivalent to multiplying by $1$."},{"type":"hyper_latex","block":{"nodes":[{"latex":"$$= \\frac{\\htmlData{anchor=num-fact}{(\\cos x - \\sin x)(\\cos x + \\sin x)}}{\\cos x + \\sin x}$"},{"latex":"$$= \\frac{\\htmlData{anchor=num-exp}{\\cos^2 x - \\sin^2 x}}{\\cos x + \\sin x}$"}],"highlights":[{"color":"purple","style":"text-color","anchor":"num-fact"},{"color":"purple","style":"text-color","anchor":"num-exp"}],"connections":[{"to":"num-exp","from":"num-fact","color":"purple","label":"expand","style":"solid"}]},"order":2,"title":"Expand the numerator","description":"Next, simplify the numerator by expanding the product. Notice that this is a difference of two squares in the form $(a - b)(a + b) = a^2 - b^2$, where $a = \\cos x$ and $b = \\sin x$."},{"type":"text","order":3,"title":"Apply double-angle identity","content":"Now we recall the double-angle formula for cosine from our trigonometric identities:\n\n$$\\cos 2x \\equiv \\cos^2 x - \\sin^2 x$$\n\nSubstituting this into the numerator gives us:\n\n$$\\frac{\\cos 2x}{\\cos x + \\sin x}$$"},{"type":"text","order":4,"title":"State final conclusion","content":"This matches the left-hand side (LHS) of our original identity:\n\n$$\\text{RHS} \\equiv \\text{LHS}$$\n\nThus, we have successfully proven that:\n\n$$\\frac{\\cos 2x}{\\cos x + \\sin x} \\equiv \\cos x - \\sin 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Trigonometry (A-level only)","topicId":"cbc5211d-47b5-4b58-b978-00f0a36a6d24","topicSidebar":"$L3c","topicSlug":"trigonometry-15","topicTitle":"3.4.7 Trigonometry (A-level only)"}]}],"$L3d","$L3e"]}]

3.4.7 Trigonometry (A-level only)

Question 3

3.4.7 Trigonometry (A-level only) Questions

3.4.7 Trigonometry (A-level only)

Question 3

3.4.7 Trigonometry (A-level only) Questions