use the formula to evaluate the series 1-2+4-8+...a-10

Question

in need help

Answer 1

The series is geometric with common ratio r = -2

 

If a-10 means a₁₀, then the sum of the first 10 terms of the series is

 

a₁(1 - r^10-1)/(1-r) = 1[1 - (-2)⁹]/[1 - (-2)]

 

                                  = 171