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).