#### Answer

$\dfrac {n\left( n-1\right) }{2}$

#### Work Step by Step

$\binom{n}{n-2}=\dfrac {n!}{\left( n-2\right) !\times \left( n-\left( n-2\right) \right) !}=\dfrac {n!}{\left( n-2\right) !\times 2!}=\dfrac {\left( n-2\right) !\times \left( n-1\right) \times n}{\left( n-2\right) !\times 1\times 2}=\dfrac {n\left( n-1\right) }{2}$