Prove that (3n+1)2−(3n−1)2(3n + 1)^2 - (3n - 1)^2(3n+1)2−(3n−1)2 is always a multiple of 12, for all positive integer values of nnn.
14 exam-style questions on Edexcel IGCSE Maths Proof. Each one has a worked solution and a mark scheme showing where the marks go.