数列1,-5,9,-13,17,-21,(-1)^n-1(4n-3),

问题描述:

数列1,-5,9,-13,17,-21,(-1)^n-1(4n-3),
求s15

a15=(-1)^15-1(4*15-3)=57
S14=(1-5)+(9-13)...=7*(-4)=-28
S15=S14+a15=-28+57=29