数列AN的通项公式AN=2^N+N-1,求S8

题目

数列AN的通项公式AN=2^N+N-1,求S8

答案

S8 = a1 + a2 + . + an
= (2 + 2^2 + 2^3 + ... + 2^8) + (0 + 1 + 2 + ... + 7)
= 2 ^ 9 - 2 + 8×7 / 2
= 512 - 2 + 28
=538