Question

When the first n positive integers are added together, their sum is given by
1/2n(n+1)

Find the value of n for which the sum is 105..

Can you show me a step by step please?

Answer 1

set 1/2n(n+1) = 105
multiply by 2
n(n+1) = 210
n^2 + n - 210 = 0
(n+15)(n-14) = 0
n = -15 or 14, but n has to be a positive integer, so

there are 14 terms

Answer 2

Thankyou