Find the sum of n term of an AP whose

Question:
Find the sum of $n$ term of an AP whose $r^{\text {th }}$ term is $(5 r+1)$

Solution:

To Find: The sum of n terms of an AP

Given: The $\mathrm{r}^{\text {th }}$ term.

The $\mathrm{r}^{\text {th }}$ term of the series is given by

$t_{r}=5 r+1$

Sum of the series is given by sum upto $n$ terms of $t_{r}$

$S_{r}=\sum_{i=1}^{n} t_{r}=\sum_{i=1}^{n} 5 r+1=\frac{5 n(n+1)}{2}+n$