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Mathematical induction is pretty cool. So, in your first step, you show that the statement is true for the base case, here, you chose n = 1. You plugged in 1 for n, and cool, the statement is true. Now, what you are doing in steps two and three is to show that if you know any arbitrary case is true, the one immediately following it is also true. So, that's why you assume the statement is true for k, then show it is true for the case right after that, k + 1. So, once you finish steps two and three, you have proven that if n = k works, then so does n = k + 1. You have already shown that n = 1 works, so therefore, n = 2 also works. Since you know that n = 2 works, then n = 3 works, and so on, so you have shown that it is true for any natural n. Another one you can prove is this: Prove that 3^n - 1 is divisible by 2.