If a_1,a_2,a_3, are in A.P.

Question

If $a_1,a_2,a_3,\ldots$ are in A.P. such that $a_1+a_7+a_{16}=40$, then the sum of the first 15 terms of this A.P. is:

Accepted Answer

$3a_1+21d=40 \Rightarrow a_1+7d = \dfrac{40}{3}$.

$$S_{15} = 15(a_1+7d) = 15\times\frac{40}{3} = 200.$$

Solutions · 1