Quadratic Equations: GCSE to A Level Revision Guide
Quadratic equations made clear for GCSE and A Level. Learn factorising, completing the square and the formula with worked examples and mistakes to avoid.
Quadratic equations have a strange talent: they look familiar, then steal your confidence under timed conditions. One minute it’s a neat x2x^2x2 term and a couple of numbers, the next it’s “show that…”, “hence find…”, or a curve-sketching question that suddenly wants exact roots. If that’s you, you’re not behind. You’re just meeting quadratic equations the way exams present them: as a small idea wearing different outfits.
This guide gives you a calm, method-first way to revise quadratic equations for GCSE (foundation and higher) and A Level (Edexcel, AQA, OCR, Eduqas). You’ll see when to factorise, when to use the quadratic formula, when completing the square is the most elegant option, and how to check answers quickly.
Use it alongside YesGenie’s free revision lessons, practice questions, mark schemes, predicted papers and past papers: YesGenie Resources.
Three methods, three moods
A quick checklist for quadratic equations
When you see quadratic equations, ask these three questions before you write anything:
- Is it in standard form? Aim for ax2+bx+c=0ax^2+bx+c=0ax2+bx+c=0.
- Does it factorise nicely? Look for integer factors of acacac that sum to bbb.
- Do I need exact answers or an approximation? Exact roots often mean factorising, completing the square, or the quadratic formula. Approximations suggest the formula or calculator.
Then choose a method:
- Factorising (fastest when it works)
- Quadratic formula (works for every quadratic)
- Completing the square (best for turning points, transformations, some “show that” questions)
If you’re building your topic list, start from the general revision area and drill down: All revision resources on YesGenie.
What quadratic equations really are (and why exams love them)
A quadratic is any expression of the form ax2+bx+cax^2+bx+cax2+bx+c where a≠0a\neq 0a=0. A quadratic equation sets that expression equal to something, usually 000:
ax2+bx+c=0. ax^2+bx+c=0. ax2+bx+c=0.Solving quadratic equations means finding the xxx-values where the parabola crosses the xxx-axis. That’s why the same skill appears in different GCSE and A Level topics: graphs, algebraic manipulation, simultaneous equations, inequalities, optimisation, kinematics. The “quadratic” part is the thread.
A small but powerful habit: always rewrite into =0 \,=0\,=0 form first. It reduces mistakes and makes method choice obvious.
Solving quadratic equations by factorising
Factorising is the method that feels like a shortcut, but it’s actually a test of structure. If you can express
ax2+bx+c=(px+q)(rx+s), ax^2+bx+c=(px+q)(rx+s), ax2+bx+c=(px+q)(rx+s),then the zero product rule says:
(px+q)(rx+s)=0⇒px+q=0 or rx+s=0. (px+q)(rx+s)=0 \Rightarrow px+q=0 \text{ or } rx+s=0. (px+q)(rx+s)=0⇒px+q=0 or rx+s=0.Worked example (GCSE Higher)
Solve x2−7x+12=0x^2-7x+12=0x2−7x+12=0.
Factorise by finding two numbers that multiply to 121212 and add to −7-7−7: −3-3−3 and −4-4−4.
x2−7x+12=(x−3)(x−4). x^2-7x+12=(x-3)(x-4). x2−7x+12=(x−3)(x−4).Set each bracket to zero:
x−3=0⇒x=3, x-3=0 \Rightarrow x=3, x−3=0⇒x=3, x−4=0⇒x=4. x-4=0 \Rightarrow x=4. x−4=0⇒x=4.So the solutions are x=3x=3x=3 and x=4x=4x=4.
Worked example (GCSE Higher: leading coefficient not 1)
Solve 2x2−5x−3=02x^2-5x-3=02x2−5x−3=0.
Multiply a×c=2×(−3)=−6a\times c=2\times(-3)=-6a×c=2×(−3)=−6. Find two numbers that multiply to −6-6−6 and add to −5-5−5: −6-6−6 and 111.
Split the middle term:
2x2−6x+x−3=0. 2x^2-6x+x-3=0. 2x2−6x+x−3=0.Factor by grouping:
2x(x−3)+1(x−3)=0, 2x(x-3)+1(x-3)=0, 2x(x−3)+1(x−3)=0, (2x+1)(x−3)=0. (2x+1)(x-3)=0. (2x+1)(x−3)=0.So:
2x+1=0⇒x=−12, 2x+1=0 \Rightarrow x=-\frac{1}{2}, 2x+1=0⇒x=−21, x−3=0⇒x=3. x-3=0 \Rightarrow x=3. x−3=0⇒x=3.Factorising is a core GCSE skill, but it also stays relevant at A Level when expressions get rearranged into solvable factors.
Solving quadratic equations with the quadratic formula
The quadratic formula is the method you use when factorising is awkward or impossible over integers.
For ax2+bx+c=0ax^2+bx+c=0ax2+bx+c=0:
x=−b±b2−4ac2a. x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}. x=2a−b±b2−4ac.The discriminant Δ=b2−4ac\Delta=b^2-4acΔ=b2−4ac tells you what kind of solutions you’ll get:
- Δ>0\Delta>0Δ>0: two distinct real roots
- Δ=0\Delta=0Δ=0: one repeated real root
- Δ<0\Delta<0Δ<0: no real roots (complex roots at A Level)
Worked example (GCSE Higher / A Level starter)
Solve 3x2+2x−1=03x^2+2x-1=03x2+2x−1=0.
Here a=3a=3a=3, b=2b=2b=2, c=−1c=-1c=−1.
x=−2±22−4⋅3⋅(−1)2⋅3=−2±4+126=−2±166=−2±46. x=\frac{-2\pm\sqrt{2^2-4\cdot 3\cdot(-1)}}{2\cdot 3} =\frac{-2\pm\sqrt{4+12}}{6} =\frac{-2\pm\sqrt{16}}{6} =\frac{-2\pm 4}{6}. x=2⋅3−2±22−4⋅3⋅(−1)=6−2±4+12=6−2±16=6−2±4.So:
x=−2+46=26=13, x=\frac{-2+4}{6}=\frac{2}{6}=\frac{1}{3}, x=6−2+4=62=31, x=−2−46=−66=−1. x=\frac{-2-4}{6}=\frac{-6}{6}=-1. x=6−2−4=6−6=−1.A quiet exam advantage: if Δ\sqrt{\Delta}Δ is a tidy integer, your final answers often simplify cleanly.
Solving quadratic equations by completing the square
Completing the square is the method that feels slower at first, then suddenly becomes the most useful tool you have. It converts a quadratic into vertex form:
a(x−h)2+k. a(x-h)^2+k. a(x−h)2+k.That helps with:
- solving quadratic equations (especially when factorising is hard)
- finding turning points (GCSE graphs and A Level calculus links)
- solving inequalities and sketching parabolas
Worked example (GCSE Higher)
Solve x2+6x+1=0x^2+6x+1=0x2+6x+1=0 by completing the square.
Start with the x2x^2x2 and xxx terms:
x2+6x+1=(x2+6x+9)−9+1. x^2+6x+1=(x^2+6x+9)-9+1. x2+6x+1=(x2+6x+9)−9+1.Because (62)2=9\left(\frac{6}{2}\right)^2=9(26)2=9.
So:
x2+6x+1=(x+3)2−8. x^2+6x+1=(x+3)^2-8. x2+6x+1=(x+3)2−8.Set equal to zero:
(x+3)2−8=0⇒(x+3)2=8. (x+3)^2-8=0 \Rightarrow (x+3)^2=8. (x+3)2−8=0⇒(x+3)2=8.Take square roots:
x+3=±8=±22. x+3=\pm\sqrt{8}=\pm 2\sqrt{2}. x+3=±8=±22.So:
x=−3±22. x=-3\pm 2\sqrt{2}. x=−3±22.This is a classic example where factorising won’t give nice integers, but the exact surd answers are still straightforward.
Whatever you do to one side...
Quadratic equations in disguise (rearranging and substituting)
Exams rarely announce themselves with a clean ax2+bx+c=0ax^2+bx+c=0ax2+bx+c=0. Quadratic equations often arrive as:
- fractions
- brackets
- powers like (x+1)2(x+1)^2(x+1)2
- substitution questions in A Level
Worked example (GCSE Higher: rearrange first)
Solve 2x+1=x\frac{2}{x}+1=xx2+1=x.
Multiply both sides by xxx (with the note x≠0x\neq 0x=0):
2+x=x2. 2+x=x^2. 2+x=x2.Rearrange:
x2−x−2=0. x^2-x-2=0. x2−x−2=0.Factorise:
(x−2)(x+1)=0. (x-2)(x+1)=0. (x−2)(x+1)=0.So:
x=2 or x=−1. x=2 \text{ or } x=-1. x=2 or x=−1.Both are valid (neither is 000), so both solutions stand.
Worked example (A Level: substitution creates a quadratic)
Solve x4−5x2+4=0x^4-5x^2+4=0x4−5x2+4=0.
Let u=x2u=x^2u=x2. Then:
u2−5u+4=0. u^2-5u+4=0. u2−5u+4=0.Factorise:
(u−1)(u−4)=0⇒u=1 or u=4. (u-1)(u-4)=0 \Rightarrow u=1 \text{ or } u=4. (u−1)(u−4)=0⇒u=1 or u=4.Now substitute back:
- If x2=1x^2=1x2=1, then x=±1x=\pm 1x=±1.
- If x2=4x^2=4x2=4, then x=±2x=\pm 2x=±2.
So x∈{−2,−1,1,2}x\in\{-2,-1,1,2\}x∈{−2,−1,1,2}.
That pattern -- reduce to a quadratic, solve, then interpret solutions -- is everywhere in A Level maths.
Checking solutions quickly (the habit that saves marks)
A surprising number of lost marks on quadratic equations come from one sign error early on. Checking doesn’t need to be long.
Pick one solution and substitute into the original equation. If you’re short on time, substitute into your rearranged =0 \,=0\,=0 form.
For example, from 3x2+2x−1=03x^2+2x-1=03x2+2x−1=0, check x=−1x=-1x=−1:
3(−1)2+2(−1)−1=3−2−1=0. 3(-1)^2+2(-1)-1=3-2-1=0. 3(−1)2+2(−1)−1=3−2−1=0.It works. Do the same for x=13x=\frac{1}{3}x=31:
3(13)2+2(13)−1=3⋅19+23−1=13+23−1=0. 3\left(\frac{1}{3}\right)^2+2\left(\frac{1}{3}\right)-1=3\cdot\frac{1}{9}+\frac{2}{3}-1=\frac{1}{3}+\frac{2}{3}-1=0. 3(31)2+2(31)−1=3⋅91+32−1=31+32−1=0.That’s 20 seconds to protect several marks.
Common mistakes with quadratic equations
Quadratic equations are consistent. Most errors are too.
- Not rearranging to 000 first. If you solve x2=7x−12x^2=7x-12x2=7x−12 without moving terms carefully, signs go missing. Always write x2−7x+12=0x^2-7x+12=0x2−7x+12=0.
- Dropping the ±\pm± when square rooting. From (x+3)2=8(x+3)^2=8(x+3)2=8, you must write x+3=±8x+3=\pm\sqrt{8}x+3=±8. One root is half the marks gone.
- Incorrect substitution into the quadratic formula. The formula is reliable, but only if aaa, bbb, and ccc match ax2+bx+c=0ax^2+bx+c=0ax2+bx+c=0. A common slip is using bbb as positive when it’s negative in the equation.
- Factorising errors with a≠1a\neq 1a=1. For 2x2−5x−32x^2-5x-32x2−5x−3, students often guess (2x−3)(x+1)(2x-3)(x+1)(2x−3)(x+1) which expands to 2x2−x−32x^2-x-32x2−x−3, not the original. Expand once to confirm.
- Cancelling illegally when x=0x=0x=0 is possible. If you multiply or divide by xxx, state x≠0x\neq 0x=0 and check whether x=0x=0x=0 could have been a solution earlier.
- Forgetting to interpret solutions in context. In geometry or kinematics questions, a negative time or negative length might be rejected. The algebra can be correct but the final answer must make sense.
How to revise quadratic equations effectively on YesGenie
Quadratic equations improve fastest when you rotate methods across mixed questions, not when you do twenty of the same type in a row. A good routine is:
- Learn or refresh the method from a revision lesson
- Practise a small set of questions with mark schemes
- Mix in past paper questions so the topic appears in context
Start with the hub and work down into GCSE and A Level resources: YesGenie Resources. Use it to find practice questions, predicted papers, mini tests and past papers by exam board.
When you’re close to exams, predicted papers can help you practise under realistic constraints: Predicted Papers on YesGenie (open the Predicted Papers section).
If you want shorter bursts, use the mini tests to keep quadratic equations fresh between bigger papers: Mini Tests on YesGenie (open the Mini Tests section).
Closing: make quadratic equations feel predictable
Quadratic equations aren’t hard because they’re complicated. They’re hard because they’re versatile, and exams love versatile ideas. When you can look at a question and calmly choose between factorising, completing the square, and the quadratic formula, quadratic equations stop being a surprise and start being a routine.
Build that routine on YesGenie: use the free revision lessons to lock in each method, then practise with questions and mark schemes, and finally pressure-test your skills with predicted papers and past papers. Start here and work outward: YesGenie Resources. Quadratic equations are a topic where small improvements compound quickly -- and once they click, they keep paying you back across GCSE and A Level maths.