✔ 最佳答案
Let P(n) be the statement " a_n=4n-2 " for all positive integers n
When n=1,a_1=2=4*1-2. LHS=RHS. So P(1) is true
Assume that P(k) is true. i.e. a_k=4k-2
When n=k+1, a_k+1=a_k+4=4k-2+4=4k+2=4(k+1)-2
LHS=RHS. So P(k+1) is true
By M.I. for all positive integers n, P(n) is true.