例题：1,5,9,13,17···4N-3 0,1,3,6,10····______

题目

例题：1,5,9,13,17···4N-3 0,1,3,6,10····______

答案

(N-1)N/2
第一问,求通项
因
a2 - a1 = 1
a3 - a2 = 2
a4 - a3 = 3
.
an - an-1 = n-1
将上式左右两边分别相加,得
an - a1 = 1+2+3+.+(n-1)
所以
an = (n-1)n/2 +a1 = (n-1)n/2