Question

Find the sum of 12 terms of the AP 5, 8, 11, 14,...

Answer 1

Here, a = 5, d = (8 − 5) = 3 and n = 12

Using the formula, $S_n=\frac{n}{2}\left[2a+\left(n-1\right)d\right]$, we get:
$$\begin{gathered} S_{12}=\frac{12}{2}\left[2\times 5+\left(12-1\right)\times 3\right][\therefore a=5,d=3\mathrm{and}n=12] \\ =6\times \left[10+33\right]=6\times 43=258 \end{gathered}$$

Hence, the sum of the first 12 terms of the given AP is 258.