2x^2 y^2+2xy-7x^2 y^2-二分之三xy+4x^2 y^2
=-X^2Y^2+1/2XY
当X=-2,Y=1/4,
原式=-(-2)^2(1/4)^2+1/2×(-2)×1/2
=-1/4-1/2
=-3/4