Prove that \( \frac{(n-1) !}{(n-r-1) !}+r \cdot \frac{(n-1) !}{(n-r) !}=\frac{n !}{(n-r) !} \)

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Prove that \( \frac{(n-1) !}{(n-r-1) !}+r \cdot \frac{(n-1) !}{(n-r) !}=\frac{n !}{(n-r) !} \)
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