通分 a/(y-x)^2,b/(x-y)(x+y),
公分母：(x-y)²(x+y),
a/(y-x)² =a/(x-y)² =a(x+y)/(x-y)²(x+y)
,b/(x-y)(x+y)=b(x-y)/(x-y)²(x+y)