(2x-x^(-1/2))^6
展开项为 C(i,6)(2x)^(6-i) *(x^(-1/2))^(i)
=C(i,6)2^(6-i) x^(6-3i/2)
i=4,展开项为常数项
C(4,6)2^2
=15*4
=60
C(m,n)是n个数中找个m个的组合数