求导：y = ( tan x )＾3 [为什么等于3 ( sec x )＾4 - 3 ( sec x )＾2 ]

y=( tan x )＾3
y'=3( tan x )＾2*(tanx)'
=3( tan x )＾2*sec^2x
=3sin^2x*sec^2x*sec^2x
=3seC^4x*sin^2x
=3seC^4x*(1-cos^2x)
=3sec^4 x -3sec^4xcos^2x
=3sec^4 x -3sec^2 x