Find the sum of the geometric sequence series 20-10+5-5/2

= 12.5

Step-by-step explanation:

This is equivalent to

20 * (1 - 1/2 + 1/4 - 1/8 + ...)

= 20*[ (1 + 1/4 + 1/16 + ...) - (1/2) (1 + 1/4 + 1/18 + ...) ]

= 10 * (1 + 1/4 + 1/16 + ...)

= 10 * (4/3) = 40/3 or 13.333 ...

To check for a gross error, notice that 20 - 10 + 5 - 5/2 = 12.5, so we are certainly in the correct general range (suggesting that my answer is in fact correct).