设(x+1/x)^6展开式的常数项为:C(6,r)*x^(6-r)*x^(-r)=C(6,r)*x^(6-2r) , 令6-2r=0,则r=3,x^(6-2r)=1,(x+1/x)^6展开式的常数项=C(6,r)=C(6,3)=(6*5*4)/(1*2*3)=20
设(x+1/x)^6展开式的常数项为:C(6,r)*x^(6-r)*x^(-r)=C(6,r)*x^(6-2r) , 令6-2r=0,则r=3,x^(6-2r)=1,(x+1/x)^6展开式的常数项=C(6,r)=C(6,3)=(6*5*4)/(1*2*3)=20