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How to combine the fractions on the righthand side over the common denominator:


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I tried with $k!(k-1)!(n-k+1)$ but I don't know how to do it with factorial – tobyyy Apr 15 '13 at 17:43

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$$\begin{align} \color{red}{k!}\cdot \color{blue}{(k-1)!}\cdot \color{green}{(n-k)!} &= \color{red}{k(k-1)!}\cdot\color{blue}{(k-1)(k-2)!}\cdot\color{green}{\dfrac{(n-k+1)!}{n-k+1}}\\ &= \dfrac{\color{red}{k}\color{blue}{(k-1)}}{\color{green}{n-k+1}} \color{red}{(k-1)!}\color{blue}{(k-2)!}\color{green}{(n-k+1)!} \end{align}$$

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Hint: $$k!=k\cdot(k-1)!\\(k-1)!=(k+1)\cdot(k-2)!\\(n-k+1)!=(n-k+1)\cdot(n-k)!$$

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