Question

Find the sum of the given series $6+11+16+21+......+86$

Answer 1

We have,

$6+11+16+21+27+......+86$

$a=6,d=5,l=86$

We know that

$l=6+(n-1)d$

$86=6+(n-1)\times 5$

$n=17$

We know that

$S_{17}=\frac{17}{2}[12+(17-1)\times 5]$

$S_{17}=\frac{17}{2}[12+16\times 5]$

$S_{17}=\frac{17}{2}[12+80]$

$S_{17}=\frac{17}{2}\left[92\right]$

$S_{17}=782$

Hence, this is the answer.