There are three kinds of mathematical induction commonly used in senior high school: (1) the first mathematical induction (commonly used, abbreviated) (2) the second mathematical induction. The proof steps are as follows: ① Proposition P(n0) holds when n=n0(n0∈N+); ② Assuming that the proposition P(m) holds for all natural numbers M suitable for n0≤m≤k, it can be inferred that P(k+ 1) holds. According to the above two points, all natural numbers n(n≥m) and P(n) hold. (3) backward induction (also called backward induction): let p be. ② Assuming that P(h+ 1) holds, it can be inferred that P(h) holds. For all natural numbers n, the proposition P(n) holds.