Question

If the sum of first n even natural numbers is 420 find the value of n

Answer 1

I hope this may help you . Thank you

Answer 2

according to the sum,
2+4+6...........=420
the above is in A.P
In it a=2 ,d=2
sum of n even no =n/2*(2a+(n-1)d)
420=n/2*(2×2+(n-1)2)
840=n (4+2n-2)
840=2n^2+2n
n^2+n=420
n^2+n-420=0
n^2+21n-20n-420=0
n (n+21)-20 (n+21)=0
(n+21)(n-20)=0
so n=20 or -21
but n is always positive. so n =20

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