Derivative of f(x)=4cos(5x-1)

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

   1. Let $u = 5 x - 1$.

   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 $\frac{d}{d x} \left(5 x - 1\right)$:

      1. Differentiate $5 x - 1$ 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: $5$

         2. The derivative of the constant $-1$ is zero.

         The result is: $5$

      The result of the chain rule is:

      $$- 5 \sin{\left(5 x - 1 \right)}$$

   So, the result is: $- 20 \sin{\left(5 x - 1 \right)}$

2. Now simplify:

   $$- 20 \sin{\left(5 x - 1 \right)}$$

The answer is: