2(二分之一x^2-3xy-y^2)-(2x^2-7xy-2y^)
=x²-6xy-2y²-2x²+7xy+2y²
=-x²+xy
=-4²+4x(-2/3)
=-16-8/3
=-56/3=x²-6xy-2y²-2x²+7xy+2y² +2x²?为啥不是加咧……前面有负号呀