Question

What is the sum of the geometric sequence -4,24,-144, ... if there are 7 terms?

A. -159,964
B.-26,661
C. 26,661
D. 159,964

Answer 1

`S_7=-4+24-144+864-5184+31104-186624`

`S_7=-4(1-6+36-216+1296-7776+46656)`

`(-6)S_7=-4(-6+36-216+1296-7776+46656-279936)`

`S_7-(-6)S_7=-4(1+279936)`

`S_7=-\frac47*279937=-159964`