Question

What is the sum of the arithmetic sequence 3,9,15, if there are 36 terms ...?

Answer 1

n = 36

a = 3

d = 3

(n/2) (2a + (n - 1)d)

(36/2) (2(3) + (36 - 1)3)

18(6+35(3))

18 x 111 = 1998

The sum is 1998