In a cryptographic protocol, the number of decryption cycles, DDD, required for a security parameter s∈Ns \in \mathbb{N}s∈N is given by D=s2+4s+7D = s^2 + 4s + 7D=s2+4s+7.
Use algebra to prove by contradiction that if D D\,D is even, then s s\,s must be odd.
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.