当x=3,y=-3分之1时,求代数式3x的平方y-[2xy的平方-4（1/2xy-3/4x的平方y）+xy]+3x y的平方

解
3x²y-[2xy²-4(1/2xy-3/4x²y)+xy]+3xy²
=3x²y-(2xy²-2xy+3x²y+xy)+3xy²
=3x²y-(2xy²-xy+3x²y)+3xy²
=(3x²y-3x²y)+(3xy²-2xy²)+xy
=xy+y²
=3×(-1/3)+(-1/3)²
=-1+1/9
=-8/9