Derivative of cos(9x-10)

1. Let $u = 9 x - 10$.

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 $\frac{d}{d x} \left(9 x - 10\right)$:

   1. Differentiate $9 x - 10$ term by term:

      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: $9$

      2. The derivative of the constant $\left(-1\right) 10$ is zero.

      The result is: $9$

   The result of the chain rule is:

   $$- 9 \sin{\left(9 x - 10 \right)}$$

4. Now simplify:

   $$- 9 \sin{\left(9 x - 10 \right)}$$

The answer is:

$$- 9 \sin{\left(9 x - 10 \right)}$$