（1）3x²-[5x-(1/2x-3)+2x²]=3x²-(5x-1/2x+3+2x²)=3x²-9/2x-3-2x²=x²-9/2x-3（2）2（x+y)-1/4(x+y)+2/3(x-y)=2x+2y-1/4x-1/4y+2/3x-2/3y=29/12x+13/12y