(1)把4又1/4拆成4+1/4,x^2+y^2-4x+y+4+1/4=0 (x-2)^2*(y+1/2)^2=0 x=2 y=-1/2 所以y^2-x+3xy=-19/4（2）可将a(a-1)-(a^2-b)=-2,化简为b-a=-2再将目标式子化为a(a-b)+b^2/2就可化为 2a+b^2/2,再将b=a-2代入2a+b^2/2=2...