求x=tany的反函数y=arctanx的导数
y'(x)=(arctanx)'(对x求导)=1/(tany)'(对y求导)=1
/sec^2(y)=1/(1+tan^2(y))=1/1+x^2.
1、为何：x是自变量,此式是对x求导.1/(tany)'(对
y求导)呀
2、tany中y是何量