JAMB 1992 · UME · Q20

For an arithmetic sequence, the first term is 2 and the common difference is 3. Find the sum of the first 11 terms.

Worked solution (try it first)

Use $S_n = \frac n2\big(2a + (n - 1)d\big)$ with $a = 2$ , $d = 3$ and $n = 11$ .
The bracket is $2 \times 2 + 10 \times 3 = 4 + 30 = 34$ .
So $S_{11} = \frac{11}{2} \times 34 = 187$ , option B.