Derivative of 1/2cosx²-sin²x

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

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

      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:

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

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

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

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

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

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

         1. The derivative of sine is cosine:

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

         The result of the chain rule is:

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

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

   The result is: $- 3 \sin{\left(x \right)} \cos{\left(x \right)}$

2. Now simplify:

   $$- \frac{3 \sin{\left(2 x \right)}}{2}$$

The answer is:

$$- \frac{3 \sin{\left(2 x \right)}}{2}$$