Answer
$sech \theta \tanh \theta (\ln sech \theta)$
Work Step by Step
Given: $y=sech \theta (1-\ln sech \theta)$
As we know, $\dfrac{d}{dx} (sech x)=-(sech x \tanh x)$
Thus, $\dfrac{d y}{d \theta}=sech \theta[\dfrac{-1}{sech \theta}( -sech \theta \tanh \theta)]+(1-\ln sech \theta)=(sech \theta \tanh \theta)[1-(1-\ln sech \theta)]$
or, $=sech \theta \tanh \theta (\ln sech \theta)$
