The textbook 2-2, an elective course in senior two mathematics, has 96 pages and 96 pages, the first three small questions of the first question in group A, and the answer to the second question in group B. Proof 1. When n= 1, left = 1, right =1/2 *1* (1+) =1,so left = right, so when n. +k= 1/2K(K+ 1), 1+2+3+,+k+(k+1) =1/2k (k+1.