Question

Evaluate 7 sigma n=1 2(-2)^n-1

Answer 1

Okay I think you mean the sum of the first 7 terms of the geometric sequence:

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

The sum of any geometric sequence is:

s(n)=a(1-r^n)/(1-r)  in this case:

s(7)=2(1--2^7)/(1-7)

s(7)=-43

Answer 2

\[\sum_{n=1}^{7} 2(-2)^{n-1}\]