$\"\"$ \ \

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The permutation formula is $\"\"$ .

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To prove: $\"\"$ when $\"\"$ . $\"\"$

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$\"\"$ (Permutation formula)

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$\"\"$ (Substitute n for r) \ \

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$\"\"$ (Subtract denominator: $\"\"$ )

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$\"\"$ (Substitute 1 for 0!) \ \

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= n!

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$\"\"$

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$\"\"$ when $\"\"$ is proved.