Find the sum of each of the geometric series given below. For the value of the sum, enter an expression that gives the exact value, rather than entering an approximation.

A. $-15 + 5 - {5\over 3} + {5\over 9} - {5\over 27} + {5\over 81} - \cdots =$

B. $\sum\limits_{n=4}^{17} \left({1\over 2}\right)^n =$