已知等比数列｛an｝中,a1=-1,q=-1/2,n=6,求Sn 怎样解？

Question

如题所述

Answer 1

a1=-1,q=-1/2,n=6,

an=a1+(n-1)q=-1+5x(-1/2)=-7/2
Sn=(a1+an)xn/2=[-1+(-7/2)] x 6 /2=-9/2 x3=-27/2