[tex]\\\\nCk=C^k_n=\left(\begin{array}{cc}k\\n\end{array}\right)=\dfrac{n!}{k!(n-k)!}\\\\9C4=C^4_9=\left(\begin{array}{cc}9\\4\end{array}\right)=\dfrac{9!}{4!(9-4)!}=\dfrac{9!}{4!\cdot5!}=\dfrac{5!\cdot6\cdot7\cdot8\cdot9}{2\cdot3\cdot4\cdot5!}\\\\=7\cdot2\cdot9=126\\----------------------------------\\nPk=P^n_k=\left(\begin{array}{cc}n\\k\end{array}\right)=\dfrac{n!}{(n-k)!}\\\\6P4=P^6_4=\dfrac{6!}{(6-4)!}=\dfrac{6!}{2!}=\dfrac{2!\cdot3\cdot4\cdot5\cdot6}{2!}=3\cdot4\cdot5\cdot6=360[/tex]
