JAMB 2002 · UME · Q32

Sequences & series (AP, GP)

If the 9th term of an A.P. is five times the 5th term, find the relationship between $a$ and $d$ .

Worked solution (try it first)

The 9th term is $a + 8d$ and the 5th term is $a + 4d$ .
So $a + 8d = 5(a + 4d) = 5a + 20d$ .
Collect terms: $0 = 4a + 12d$ .
Divide by 4: $a + 3d = 0$ , option B.

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