Question

what is n choose 2?

Answer 1

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Answer 2

That is the number of possibilities to choose two items out of n if you don't distinguish between different orders. The formula is:
\[n\;\text{choose}\;2 = \binom n 2 = \frac{n!}{2!(n-2)!} = \frac{n\cdot (n-1)}{2}\]