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