Find the sum of the first 18 terms of the series 3, 6, 9,…, 36.

Explanation

3, 6, 9,..., 36.

a = 3, d = 3, i = 36, n = 18

S_n = \(\frac{n}{2}\) [2a + (n - 1)d

S₁₈ = \(\frac{18}{2}\) [2 x 3 + (18 - 1)3]

= 9[6 + (17 x 3)]

= 9 [6 + 51] = 9(57)

= 513

There is an explanation video available .