[(x+2y)²-(x+y)(3x-y)-5y]/2x=[(x²+4xy+4y²)-(3x²-xy+3xy+y²)-5y]/2x=(-2x²+2xy+3y²-5y)/2x=-x+y+3y²/2x-5y/2x将x=-2,y=1/2代入上式得：[(x+2y)²-(x+y)(3x-y)-5y]/2x=...