Question

If the sum of first n even natural number is
240. find the value of n.



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Answer 1

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Genius6353

11.08.2017

Math

Secondary School

+13 pts

Answered

If sum of first n even natural number is 420 then find n

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Answers

Even natural no:s form an A.P :-

2,4,6,8 .....

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

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Answer 2

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