Question

The sum S of the first n natural number is given by relation S=n(n-1)/2 find 'n' if the sum is 120

Answer 1

Answer:

Hope it will help you mate ❤️

Answer 2

Question:

The sum S of the first n natural numbers is given by the relation s=n(n+1)/2find n if the sum is 120

Answer:

n is 15

Solution:

Given that,

The sum S of the first n natural number is given by relation:

$s = \frac{n(n+1)}{2}$

Given that,

sum = 120

$120 = \frac{n(n+1)}{2}\\\\n(n+1) = 240\\\\n^2 + n = 240\\\\n^2 + n - 240= 0$

Split the middle term

$n^2+16n-15n-240=0\\\\n(n+16) - 15(n+16) = 0\\\\(n-15)(n +16) = 0\\\\n = 15\\\\n = -16$

Ignore negative value

n = 15

Thus n is 15

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