原式=1/2xy-{1/2xy-[1/3x^2y-x^2y+xy]+1/3x^2y} &&去小括号=1/2xy-{1/2xy-[-2/3x^2y+xy]+1/3x^2y}=1/2xy-{1/2xy+2/3x^2y-xy+1/3x^2y} &&去中括号=1/2xy-{-1/2xy+x^2y}=1/2xy+1/2xy-x^2y &&去大括号=xy-x^2y...