Prove that (5n+3)2−(5n−3)2(5n + 3)^2 - (5n - 3)^2(5n+3)2−(5n−3)2 is always a multiple of 20, for all positive integer values of nnn.
56 exam-style questions on Edexcel GCSE Maths Proof. Each one has a worked solution and a mark scheme showing where the marks go.