#### Answer

$-\dfrac {1}{2\sqrt {1-x}\times \left( \cos ^{2}\sqrt {1-x}\right) }$

#### Work Step by Step

$y=\tan \sqrt {1-x}\Rightarrow y'=\dfrac {1}{\cos ^{2}\left( \sqrt {1-x}\right) }\times \left( \dfrac {d}{dx}\sqrt {1-x}\right) =-\dfrac {1}{2\sqrt {1-x}\times \left( \cos ^{2}\sqrt {1-x}\right) }$