在(x-1/x)的9次方的二项展开式中,x³的系数为

如题所述

(x-1/x)^9
=(x^2-1)^9 / x^9
coef. of x^3 : (x-1/x)^9
=coef. of x^12 : (x^2-1)^9
=(9C6) (-1)^3
=-84
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