Answer
$-2x \sin (x^2-1)$
Work Step by Step
We have $f(x)=\cos (x^2-1)$
We differentiate both sides with respect to $x$.
$f'(x)= \dfrac{d}{dx}[\cos (x^2-1)]$
or, $f'(x)=-\sin (x^2-1) \dfrac{d}{dx}[(x^2-1)]=-2x \sin (x^2-1)$
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