Tetrahedral number

Statement

The $n$-th tetrahedral number (cannonballs in a tetrahedral pile) is $\mathrm{Tet}_n=\dfrac{n(n+1)(n+2)}{6}$. Compute $\mathrm{Tet}_9$.

Solution

$\mathrm{Tet}_9=\frac{9\cdot 10\cdot 11}{6}=\frac{990}{6}=\boxed{165}.$