adjacent it right- you need the chain rule.
[tex]\frac{dy}{dx}= \frac{dy}{du}\frac{du}{dx}[/tex]
First you are differentiating [itex]cos(2\theta)[/itex] with respect to [itex]\theta[/itex], not "[itex]2\theta[/itex]" so you need to multiply by the derivative of [itex]2\theta[/itex] with respect to [itex]\theta[/itex].
Second you are differentiating [itex]sin(x^2)[/itex] with respect to [itex]\theta[/itex], not "[itex]\theta^2[/itex]" so you need to multiply by the derivative of [itex]\theta^2[/itex] with respect to [itex]\theta[/itex].
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