y=arccosx的倒数为 y=-1/根号1-x^2.
所以
y=【arccos(2/x)】的导数为
y'=2/x^2/(根号1-4/x^2)=2/根号x^4-4x^2
2.y'=[1/(secx+tanx)]*(secxtanx+(secx)^2)
=secx