1、公分母：12X^2Y,
1/3X^2=4Y/(12X^2Y),
5/12XY=5X/(12X^2Y)
2、公分母：X(X+1)(X-1)
1/X^2+X=(X-1)/［X(X+1)(X-1)］,
1/(X^2-X)=(X+1)/［X(X+1)(X-1)］,
3、公分母：abc
c/ab=c^2/abc,
a/bc=a^2/abc,
b/ac=b^2/abc.