x^2(a-1)+y^2(1-a)=x^2(a-1)-y^2(a-1)=(a-1)(x^2-y^2)=(a-1)(x-y)(x+y)
x^2(a-1)+y^2(1-a)=x^2(a-1)-y^2(a-1)=（a-1)(x^2-y^2)=(a-1)(x+y)(x-y)
