The sum of first n terms of an A.P is 5n2 + 3n.


The sum of first n terms of an A.P is 5n2 + 3n. If its mth terms is 168, find the value of m. Also, find the 20th term of this A.P.


$S_{n}=5 n^{2}+3 n$

We know


$\therefore a_{n}=5 n^{2}+3 n-5(n-1)^{2}-3(n-1)$

$a_{n}=10 n-2$



$\Rightarrow 10 m-2=168$

$\Rightarrow 10 m=170$

$\Rightarrow m=17$


Leave a comment


Click here to get exam-ready with eSaral

For making your preparation journey smoother of JEE, NEET and Class 8 to 10, grab our app now.

Download Now