Question

The formula of the sum of first n natural numbers is S = n(n+1)/2. If the sum of first n natural number is 325, find n.

Answer 1

S=n(n+1)/2
325=(n^2+n)/2
325×2=n^2+n
650=n^2+n
n^2+n-650=0
n^2+26n-25n-650=0
(n^2+26n)(-25n-650)=0
n(n+26)-25(n+26)=0
(n+26)(n-25)=0
n+26=0. n-25=0
n=-26. n=25
× not possible
as n cannot
be negative

Ans: 25

Answer 2

Your answer is in the attachment.

hope it helps!

thank you.