NECO 2022 · Paper 1 · Q11

Sequences & series (AP, GP)

The $n$ th term of an Arithmetic Progression (A.P.) is defined by $T_n = \left(2n + \frac{1}{2}\right)$ . Find the sum of the first ten terms of the A.P.

Worked solution (try it first)

The first term is $T_1 = 2 + \frac{1}{2} = 2.5$ and the tenth is $T_{10} = 20 + \frac{1}{2} = 20.5$ .
Use $S_n = \frac{n}{2}(a + l)$ : $S_{10} = 5 \times (2.5 + 20.5)$ .
So $S_{10} = 5 \times 23 = 115$ , option D.

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