JAMB 2003 · UME · Q13

Sequences & series (AP, GP)

The sum of the first $n$ terms of an arithmetic progression is 252. If the first term is $-16$ and the last term is 72, find the number of terms.

Worked solution (try it first)

When you know the first and last terms, $S_n = \frac n2(a + l)$ .
So $\frac n2(-16 + 72) = 252$ , which is $28n = 252$ .
Divide both sides by 28: $n = 9$ , option B.

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