利用复合函数的求导法则：y= f(u),u= u(x),则 f ' (x) = f '(u) * u'(x)
y = cot ( lnx),设：y = cot u,u= lnx
y ' = (cotu)' = ( sinu/ cosu) ' = [ (sinu)' *cosu - sinu * ( cosu)' ]/ (cosu)^2
= (cos^2u + sin^2u) /( cos^2u) = 1/ cos^2u
u ' = (lnx)' =1/x
所以：y ' = 1/ ([cos^2( lnx)] * x)