（3/2x^2-5xy+y^2）-[-3xy+2(1/4x^2-xy)+2/3y^2]=3/2x^2-5xy+y^2+3xy-1/2x^2+2xy-2/3y^2=3/2x^-1/2x^2-5xy+3xy+2xy+y^2-2/3y^2=x^2+1/3y^2
