(1).将y/x²-xy,x/2y-2x通分后,分别是2y/2x(x-y),-xy/2x(x-y)
(2).把a/a-b,b/a+b,2/b²-a²的分母化为b²-a²后,三个分式的分子之和为b²-a²-2ab+2
通分：(3)12/m²-9=36/[3(m+3)(m-3)]
,4/9-3m=-4(m+3)/[3(m+3)(m-3)]
(4)1/x-2=-(2+x)/(4-x²)
,1/4-x²=1/4-x²
(5)a+2/a²+2a-3=(a+2)/(a+3)(a-1)
2/1-a=-2(a+3)/(a+3)(a-1)；
(6)2/x+2=2(x-2)²/[(x+2)(x-2)²]
,x+2/x²-4x+4=(x+2)²/[(x+2)(x-2)²]