# J, <s>10</s> 9 Bytes

<s>Triangular number of bytes as well :)</s>

-1 Byte thanks to @FrownyFrog

    0=1|2!inv

### Explanation:

        2!       | n choose 2
          inv    | Inverse
    0=1|         | Test if it's an integer

Could have been 7 bytes if the truthy/falsy values didn't have to be constant.

Works since n choose 2 is n!/2!(n-2)! = n*(n-1)/2

I don't know of any shorter ways to test for integers, previously I had been using `(=<.)`