（xy-2/3y-2/3)-(1/2x-1/2xy+1)
=xy-2/3y-2/3-1/2x+1/2xy-1
=(1+1/2)xy-2/3y-1/2x-(2/3+1)
=3/2xy-2/3y-1/2x-5/3
当x=2/3,y=3/4时,原式=3/2*2/3*3/4-2/3*3/4-1/2*2/3-5/3
=3/4-1/2-1/3-5/3
=-7/4.