Question

The sum S of the first n natural numbers is given by the relation s=n(n+1)/2find n if the sum is 120

Answer 1

Answer:15

Step-by-step explanation:S=n(n+1)/2

If S=120

Then,n(n+1)/2=120

n^2+n=240

n^2+n-240=0

Comparing with ax^2+bx+c=0

a=1

b=1

c=-240

D =b^2-4ac

D=1+4*240

D=961

√D=31

n =-b+-√D/2a

n= -1+-31/2

n=-16 or. 15

n can't be negative

So n=15