通分:1/(x-2) ,2/(3x+6) ,3/(x^3-4x) ,
公分母是3x(x-2)(x+2),
1/(x-2)=3x(x+2)/[3x(x-2)(x+2)],
2/(3x+6)=2x(x-2)/[3x(x-2)(x+2)],
3/(x^3-4x)=9/[3x(x-2)(x+2)].