# Find the sum of $n$ terms of an $A.P.$ whose $k^{th}$ term is $5k+1$
$\begin{array}{1 1} =\large\frac{n(5n+1)}{2} \\=\large\frac{n(5n+6)}{2} \\=\large\frac{n(5n-7)}{2} \\ =\large\frac{n(5n+7)}{2} \end{array}$