Use proof by contradiction to show that for all x∈Rx \in \mathbb{R}x∈R
x4+100≥20x2 x^4 + 100 \ge 20x^2 x4+100≥20x2Show that the following statement is not true.
"The sum of three consecutive even integers is always a multiple of 12."
107 exam-style questions on OCR A Level Maths 1.1 Proof, covering 1.1.1 Structure of mathematical proof, 1.1.2 Logical connectives, 1.1.3 Disproof by counter example, 1.1.4 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.