Find how many terms of the following series are needed to make the given sum:

64+62+60 ... = 0

The sum of a series is calculated with the following formula:

n/2 (2a + (n-1) d) where n is the number of terms, a is the first term and d is the difference between the terms.

so for this series:

n/2 (2*64 + (n-1) * - 2) = 0

n (130-2n) = 0

n=0 (not valid because n<1) n=65

so the 65th term gives the sum 0