Thanks in advance!

Question

+1

578

1

+53

For what positive integer $n$ is $\frac{1+2+3+\ldots + n}{2n} = 10$ true?

Thanks

-PharaoCarl

PharaoCarl May 13, 2020

CPhill · Answer 1 · 2020-05-13T05:54:37

The numerator can be written as the sum

(n) (n + 1) / 2 so we have

(n)(n + 1)

________ = 10 simplify

2 * 2n

n + 1

_____ = 10 multiply both sides by 4

4

n + 1 = 40

n = 39