Prove that (5k+2)2−(5k−2)2(5k + 2)^2 - (5k - 2)^2(5k+2)2−(5k−2)2 is always a multiple of 40, for all positive integer values of kkk.
56 exam-style questions on Edexcel GCSE Maths Proof. Each one has a worked solution and a mark scheme showing where the marks go.