Question

find n such that C(n,2)=91

Answer 1

No such n exists.

Answer 2

n = 14

Answer 3

Whoops, haha you're right.

Answer 4

That's because C(n, 2) = n(n-1)/2. So you want to solve n(n-1)/2 = 91.
But then that’s n(n-1) = 182. But factors of 182 are just 2*7*13. So we can see 182 = 13*14, so n = 14.

Answer 5

good.......

Answer 6

but something u know not by calculation, by your own experience.......