permutaions n combinations

where to apply coefficient of (x1+x2+x3+......) and how

Dear siva,

we use multimonial theorom in permutation and combination in two cases

1) when we have to find number of ways of dividing n identical items among r persons then we will find the coefficient

of x^r in the expansion of (1+x +x² +x^3 ........ xⁿ)^r

 2)when there are m items of one kind and n items of other and so on then number of ways of choosing r items out of these items is

    coefficient of x^r  in the expansion   (1+x +x² ........................xⁿ)(1+x +x² ........................x^m)   and so on

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