Question

The sum of first n natural numbers is given
by the formula S=n(n+1)/2
If the sum of
first x natural numbers is 66, find the value
of x.​

Answer 1

Answer:

S=n(n+1)/2

Sx=x(x+1)/2

66=x(x+1)/2

66×2=x^2+x

132 = x^2+x

x^2+x-132=0

x^2+11x-12x-132=0

x(x+11) - 12(x+11) = 0

(x+11) (x-12)=0

x+11=0 or x-12=0

x=-11 or x=12

Since natural no. donot have -ve sign value of x=12