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1.1 Proof

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Question 92

Expand and simplify $x + y)^2$ .

[1]

Sarah claims that for any non-zero real number $k$ , the sum of $k$ and its reciprocal, $\frac{1}{k}$ , is always greater than or equal to 2.

Sarah tests two values: When $k = 3$ , $\frac{1}{3} = 3.\dot{3} \ge 2$ . When $k = 1$ , $\frac{1}{1} = 2 \ge 2$ .

Provide a counter-example to show that Sarah's claim is incorrect.

[2]

Given that $x$ and $y$ are distinct positive real numbers, use proof by contradiction to prove that:

\frac{x}{y} + \frac{y}{x} > 2

[4]

Markscheme

1.1 Proof Questions

A Level
Maths
1.1 Proof

132 exam-style questions on OCR (MEI) A Level Maths 1.1 Proof, covering 1.1.1 Structure of mathematical proof, 1.1.2 Disproof by counter example, 1.1.3 Proof by contradiction (A-level only), and 1.1 Proof. Each one has a worked solution and a mark scheme showing where the marks go.

1.1 Proof

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1.1 Proof

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Question 92

Expand and simplify $x + y)^2$ .

[1]

Sarah claims that for any non-zero real number $k$ , the sum of $k$ and its reciprocal, $\frac{1}{k}$ , is always greater than or equal to 2.

Sarah tests two values: When $k = 3$ , $\frac{1}{3} = 3.\dot{3} \ge 2$ . When $k = 1$ , $\frac{1}{1} = 2 \ge 2$ .

Provide a counter-example to show that Sarah's claim is incorrect.

[2]

Given that $x$ and $y$ are distinct positive real numbers, use proof by contradiction to prove that:

\frac{x}{y} + \frac{y}{x} > 2

[4]

Markscheme

1.1 Proof Questions

A Level
Maths
1.1 Proof