JAMB 2011 · UTME · Q21

Find the sum of the first 18 terms of the series $3, 6, 9, \ldots$

Worked solution (try it first)

This is an A.P. with $a = 3$ and $d = 3$ .
Use $S_n = \frac n2\big(2a + (n - 1)d\big)$ with $n = 18$ : the bracket is $6 + 17 \times 3 = 57$ .
So $S_{18} = 9 \times 57 = 513$ , option B.