Question: 5^n-1 is divisible by 4?
Answer: Assume True: 5n – 1 is divisible by 4. This means that 5n – 1 = 4a for an integer a. Then Prove: 5n+1 – 1 is divisible by 4. This means to prove that 5n+1 – 1 = 4b, for some integer b.