(a+b)^n=sum_{k=0}^nleft(begin{matrix}nkend{matrix}right)a^{n-k}b^kleft(begin{matrix}nkend{matrix}right)=frac{n!}{k!(n-k)!}" title="(a+b)^n=sum_{k=0}^nleft(begin{matrix}nkend{matrix}right)a^{n-k}b^kleft(begin{matrix}nkend{matrix}right)=frac{n!}{k!(n-k)!}" alt="(a+b)^n=sum_{k=0}^nleft(begin{matrix}nkend{matrix}right)a^{n-k}b^kleft(begin{matrix}nkend{matrix}right)=frac{n!}{k!(n-k)!}" />