Question

sum of series 1+2+3+_______ +n is: a) n(n+1) /2, b) n(n+1) c) n+1(n+2) /2 ,d) none of these.?

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Answer 1

Answer:

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Answer 2

EXPLANATION

• GIVEN

Sum of series = 1 + 2 + 3 + .....n terms

let first term = a = 1

common difference = d = b - a

2 - 1 = 1

sum of n terms

$sn \: = \frac{n}{2}(2a \: + (n - 1)d)$

$sn \: = \frac{n}{2}(2(1) + (n - 1)1$

$sn \: = \frac{n}{2}(2 + n \: - 1)$

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

Hence,

option [A] is correct