Find the sum of the first 11 terms of the geometric sequence

Find the sum of the first 11 terms of the geometric sequence. Use the formula for the sum of the first n terms of a geometric sequence 5, -20, 80, 320. The sum of the first 11 terms of the geometric sequence is. (Simplify your answer)

Solution

5, -20 , 80 , -320 ...

common ratio r = -20 / 5 = - 4

applying sum of geometric sequence formula

sum = a ( 1- r^n ) / (1-r)

sum = 5 ( 1 - (-4)^11) / ( 1+ 4)

sum of 11 terms = 4194305