The sum of first even natural no. is given by S=n(n+1) find 'n' if the sum is420
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420 = n²+n
n²+n-420 = 0
or, n²+21n - 20n -420= 0
n(n+21) - 20(n+21) = 0
(n+21) (n-20) = 0
n = 20 and n= -21
take only positive value
n= 20
n²+n-420 = 0
or, n²+21n - 20n -420= 0
n(n+21) - 20(n+21) = 0
(n+21) (n-20) = 0
n = 20 and n= -21
take only positive value
n= 20
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