If the sum of first n even natural number is
240. find the value of n.
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a = 2 , d = 2 , Sn = 420
Sn = n/2 [ 2a + (n-1)d]
420 = n/2 [ 4 + 2n - 2 ]
840 = n [ 2 + 2n ]
2n²+ 2n = 840
2 [ n² + n ] = 840
n² + n = 420
n² + n - 420 = 0
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n = -b ± √ b² - 4ac / 2a
= -1 ± √1 - 4 x 1 x -420 = 2
= -1 ± √1681 / 2
= -1 ± 41 /2
= -1 + 41/2 = 40/2 = 20
n = 20
I think your question is wrong and it is 420 not 240
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