Derivative of cos^2x-cos2x

1. Differentiate $\cos^{2}{\left(x \right)} - \cos{\left(2 x \right)}$ term by term:

   1. Let $u = \cos{\left(x \right)}$.

   2. Apply the power rule: $u^{2}$ goes to $2 u$

   3. Then, apply the chain rule. Multiply by $\frac{d}{d x} \cos{\left(x \right)}$:

      1. The derivative of cosine is negative sine:

         $$\frac{d}{d x} \cos{\left(x \right)} = - \sin{\left(x \right)}$$

      The result of the chain rule is:

      $$- 2 \sin{\left(x \right)} \cos{\left(x \right)}$$

   4. The derivative of a constant times a function is the constant times the derivative of the function.

      1. Let $u = 2 x$.

      2. The derivative of cosine is negative sine:

         $$\frac{d}{d u} \cos{\left(u \right)} = - \sin{\left(u \right)}$$

      3. Then, apply the chain rule. Multiply by $\frac{d}{d x} 2 x$:

         1. The derivative of a constant times a function is the constant times the derivative of the function.

            1. Apply the power rule: $x$ goes to $1$

            So, the result is: $2$

         The result of the chain rule is:

         $$- 2 \sin{\left(2 x \right)}$$

      So, the result is: $2 \sin{\left(2 x \right)}$

   The result is: $- 2 \sin{\left(x \right)} \cos{\left(x \right)} + 2 \sin{\left(2 x \right)}$

2. Now simplify:

   $$\sin{\left(2 x \right)}$$

The answer is:

$$\sin{\left(2 x \right)}$$