应该是通分吧
（1）2c/bd与3ac/4b
公分母：4bd
2c/bd=2c*4/4bd=8c/4bd
3ac/4b=3ac*d/4bd=3acd/4bd
(2)2xy/[x+y]²与x/x²-y²
公分母：（x+y）^2*(x-y)
2xy/[x+y]²=2xy*(x-y)/[(x+y)^2*(x-y)]
x/x²-y²=x/[(x+y)(x-y)]=x*(x+y)/[(x+y)^2*(x-y)]