根号【3?】cos30°-2sin45°+tan45°·cos60°
=3/2-根号2+1/2=2-根号2
x/y=2/3,求
3x-y/x+2y=（3x/y-1)/(x/y+2)=(2-1)/(2/3+2)=1/(8/3)=3/8