(x^3-2/x)^8展开式中x^8的系数为?

题目

(x^3-2/x)^8展开式中x^8的系数为?

答案

C8(r)*(x³)^r*(-2/x)^(8-r)
=C8(r)*x^(3r-8+r)*(-2)^(8-r)
4r-8=8
r=4
x^8的系数为:
C8(4)*(-2)^4
=70*16
=1120