-(a-2)(3-a)=(a-2)(a-3)
所以(a-2)/(a-3)=(a-2)^2/(a-2)(a-3)
(a+1)/-(a-2)(3-a)=(a+1)/(a-2)(a-3)
y-x=-(x-y)
公分母ab(x-y)
x/a(x-y)=bc/ab(x-y)
y/b(y-x)=-ax/ab(x-y)