Question

(x-1/2x)^9 展开式中x^3的系数等于?

Answer 1

Tr+1=C9(r)*x^(9-r)*(-1/2x)^r=C9(r)*(-1/2)^r*x^(9-r-r)

9-r-r=3

r=3

系数是：C9（3）*（-1/2）^3=-10.5