Question

The sum of first n even natural numbers is given by s= n(n+1) Find n if the sum is 420​

Answer 1

Answer:

20

Step-by-step explanation:

s = n^2+n

420 = n^2+n

n^2+n-420 = 0

n^2+21n-20n-420 = 0

n(n+21)-20(n+21) = 0

(n-20)(n+21) = 0

n-20 = 0 or n+21 = 0

n = 20 or n = -21

n can't be negative

therefore n = 20