Answer
$sech \theta \tanh \theta (\ln sech \theta)$
Work Step by Step
Since, $\dfrac{d}{dx} (sech x)=-sech x \tanh x$
As we are given that $y=sech \theta (1-\ln sech \theta)$
Then, on differentiating , we have:
$\dfrac{dy}{d \theta}=sech \theta[\dfrac{-1}{sech \theta}( -sech \theta \tanh \theta)]+(1-\ln sech \theta)=(sech \theta \tanh \theta)[1-(1-\ln sech \theta)]=sech \theta \tanh \theta (\ln sech \theta)$
