若A=x2-xy-y2,B=-2xy-3y2,则2A-3B=_____
问题描述:
若A=x2-xy-y2,B=-2xy-3y2,则2A-3B=_____
答
=2(x^2-xy-y^2)-3(-2xy-3y^2)
=2x^2-2xy-2y^2+6xy+9y^2
=2x^2+4xy+7y^2