Question

what is the sum of an 8-term geometric series if the first term is -11, the last term is 180,224 and the common ratio is -4?

Answer 1

`a_1=-11;\ r=-4;\ a_8=180,224;\ S_8=?\\\\S_n=(a_1(1-r^n))/(1-r)\\\\S_8=(-11[1-(-4)^8])/(1-(-4))=(-11(1-65,536))/(1+4)=(-11(-65,535))/(5)\\\\=-11\cdot(-13,107)=144,177`