NECO 2023 · Paper 1 · Q14

Sequences & series (AP, GP)

The 3rd term of a geometric progression is 18 and the 6th term is 486. Find the first term.

Worked solution (try it first)

The 3rd term is $ar^2 = 18$ and the 6th is $ar^5 = 486$ .
Divide: $r^3 = 27$ , so $r = 3$ .
Then $a \times 3^2 = 18$ , so $9a = 18$ .
So $a = 2$ , option A.

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