P(m，n)=n*(n- 1)(n-2)...(n-m+ 1)=n！ /(n-m)！ Of the n elements, take the arrangement of m elements.

C(m，n)=P(m，n)/P(m，m)=n(n- 1)(n-2)...(n-m+ 1)/m！

=n！ /[(n-m)！ *m！ ]. Take out the combination of m elements from n elements.

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