JAMB 1989 · UME · Q30

Sequences & series (AP, GP)

If $-8, m, n, 19$ are in arithmetic progression, find $(m, n)$ .

Worked solution (try it first)

Four terms in an A.P. have three equal gaps between them, so $-8 + 3d = 19$ .
Then $3d = 27$ and $d = 9$ .
So $m = -8 + 9 = 1$ and $n = 1 + 9 = 10$ , option A.

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