Answer
$f'(x)=-2\theta\sin(\theta^{2})$
Work Step by Step
$f(\theta)=\cos(\theta^{2})$
Differentiate using the chain rule:
$f'(\theta)=-\sin(\theta^{2})(\theta^{2})'=-\sin(\theta^{2})(2\theta)=-2\theta\sin(\theta^{2})$
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