Chapter 12.3, Problem 71PE

For Exercises 49−72, find the sum of the geometric series, if possible. (See Example 6−8)

71. ${\displaystyle \underset{n=3}{\overset{\infty }{\sum }}4{\left(\frac{1}{2}\right)}^{n-1}}$ (Hint: Evaluate the infinite sum from $n=1$ to infinity. Then subtract the termscorresponding to $n=1$ and $n=2$ .)