就是凑,凑成分母和分子一样的再约!比如说（随便编的啊）f(x)=1+4x^2/1-4x^2
原函数为f(x)=1-4x^2+4x^2-1+1/1-4x^2
=(1-4x^2)-(1-4x^2)+1/1-4x^2
=1-1+1/1-4x^2
=1/1-4x^2
然后再求值域或是定义域就好办了