Find the sum of first 20 terms of an A.P. whose nth term is 15 - 4n.

Since, a n = 15 - 4n ∴ a 1 = 15 - 4(1) = 15 - 4 = 11 and a 20 = 15 - 4(20) = 15 - 80 = -65. We know that Hence, the sum of first 20 terms of an A.P. is -540.