约分：10x^4 y^3 z/-15x^2 y^4 z^2 =-2x^2/3yz
m^2-3m/9-m^2=-m/m+3
通分：2/3a^2与1/-6ab^2 通分后为 4b^2/6a^2b^2与-a/6a^2b^2
2mn/4m^2-9与2m-3/2m+3 通分后为 2mn/4m^2-9与4m^2-12m+9/4m^2-9