So I have the equation
What I did was simplify $\cos(2x)$ to $1-2\sin^2(x)$ by applying the double angle formula.
Here are my steps:
Apply chain rule for $-2\sin^2(x)$ = $-4\sin(x)\cos(x)$
Derivative of $-3\sin(x)$ =$-3\cos(x)$
Final result = $-4\sin(x)\cos(x)-3\cos(x)$
The correct answer is $-2\sin(2x)-3\cos(x)$.
I have a feeling I shouldn't be using the double angle formula but I'm not sure why? Simplifying using algebra is allowed before differentiating/integrating as far as I know.
Any help is appreciated!