Question

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

Answer 1

719,835 119,973 -119,973 -719,835 , these all are the right answers .

Answer 2

Hello,
The ratio of this geometric sequence is -6.

`u_(1) =-4\\ u_(2) =-4*(-6)=24\\ u_(3) =-4*(-6)^2=-4*36=-144\\ ...\ u_(7) =-4*(-6)^6=-186624\\\\ -4+(-4)*(-6)+(-4)*(-6)^2+(-4)(-6)^3+...+(-4)*(-6)^6\\\\ =-4(1+(-6)+(-6)^2+(-6)^3+...+(-6)^6)\\\\ =-4*((-6)^7-1)/((-6)-1) =(-4)/(-7)*(-279936-1)\\\\ =-159964`