To Find: The sum of 20 terms of the given AP.

Sum of n terms of an AP with first term a and common difference d is given by

Here a = x + y, n = 20, d = - 2y

⇒S = 10[2x + 2y + 19( - 2y)] = 10[2x + 2y - 38y] = 10[2x - 36y]

⇒S = 20[x - 18y]

Sum of the series is 20(x - 18y).