复合函数求导法：
y=f(u),u=x^2arccox+tanx
y'=f'(u) u'
=f'(u)[2xarccosx-x^2/√（1-x^2)+(secx)^2]
=f'(x^2arccosx+tanx)[2xarccosx-x^2/√（1-x^2)+(secx)^2]