Question

Find the sum to n terms od A.P. whose kth term is 5k + 1.

Answer 1

Here $a_k=5k+1.$

Putting k = 1 and n

$\therefore a_1=5\times 1+1=6and{a}_n-5n+1$

Now $S_n\frac{n}{2}a+l$

$\therefore S_n=\frac{n}{2}[6+5n+1]=\frac{n}{2}(5n+7)$

$[\therefore l=a_n=5n+1]$