(cotx)`=(cosx/sinx)`

=[(cosx)`sinx-cosx(sinx)`]/sin²x(商的求导公式)

=[-sinxsinx-cosxcosx]/sin²x

=[-sin²x-cos²x]/sin²x

=-1/sin²x

(-1/sinx)′ = 1/sinx×(sinx)′ = cosx/sinx = cotx × cscx

所以cotx的二阶导数是cotx×cscx