\"\"

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The expression is \"\".

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The number of combinations of n distinct objects taken r at a time given by \"\".

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\"\"                             (Apply Combination formula)

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\"\"                       (Substitute \"\")

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\"\"                                   (Subtract: \"\")

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Expand the factorial: \"\".

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\"\"

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\"\"                                     (Cancel common terms)

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\"\"                                                    (Multiply: \"\")

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\"\"

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The value of \"\" is \"\".