1.x²/（x+y）-x+y=x^2/(x+y)-(x^2+xy）/（x+y）+（xy+y^2)/(x+y）=（2xy+y^2)/(x+y)
2.24x^4y/a÷8x^2y^2=3x^2/a*y
3.12/a²-9-2/a-3=(12-2a)/a^2-12
4.a²/a-1-a-1=a-a-1-1=-2