evaluate the geometric series when ...

Question

a₁=4, a_n=1024, r = -2

Answer 1

I am assuming that you want to find n.

 

Since the series is geometric, a_n = a₁r^n-1

 

So, we have 1024 = 4(-2)^n-1

 

                   256 = (-2)^n-1

 

                  (-2)⁸ = (-2)^n-1    n-1 = 8

 

                                             n = 9