原是=0.2x²y-1/2xy²+3/2y²x-3/4x²y
=1/5x²y-1/2xy²+3/2xy²-3/4x²y
=(1/5-3/4)x²y+(-1/2+3/2)xy²
=(4/20-15/20)x²y+xy²
=-11/20x²y+xy²
当x=2,y=3时
原式=-11/20×2²×3+2×3²
=-11/20×4×3+2×9
=-33/5+18
=90/5-33/5
=57/5