Formula combinarilor: [tex]\frac{n!}{k!*(n-k)!}[/tex]
=> [tex]\frac{11!}{9!*(11-9)!} =\frac{11!}{9!*2!} =\frac{1*2*..*9*10*11}{1*2*..*9 * 2} =\frac{10*11}{2} =55[/tex]
[tex]\frac{11!}{2!*(11-2)!} =\frac{11!}{2!*9!} =\frac{1*2*..*9*10*11}{1*2*..*9 * 2} =\frac{10*11}{2} =55[/tex]
=> 55-55=0
