Derivative of y=(cosnx)sin4x

1. Apply the product rule:

   $$\frac{d}{d x} f{\left(x \right)} g{\left(x \right)} = f{\left(x \right)} \frac{d}{d x} g{\left(x \right)} + g{\left(x \right)} \frac{d}{d x} f{\left(x \right)}$$

   $f{\left(x \right)} = \cos{\left(n x \right)}$; to find $\frac{d}{d x} f{\left(x \right)}$:

   1. Let $u = n x$.

   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 $\frac{\partial}{\partial x} n x$:

      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: $n$

      The result of the chain rule is:

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

   $g{\left(x \right)} = \sin{\left(4 x \right)}$; to find $\frac{d}{d x} g{\left(x \right)}$:

   1. Let $u = 4 x$.

   2. The derivative of sine is cosine:

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

   3. Then, apply the chain rule. Multiply by $\frac{d}{d x} 4 x$:

      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: $4$

      The result of the chain rule is:

      $$4 \cos{\left(4 x \right)}$$

   The result is: $- n \sin{\left(4 x \right)} \sin{\left(n x \right)} + 4 \cos{\left(4 x \right)} \cos{\left(n x \right)}$

The answer is:

$$- n \sin{\left(4 x \right)} \sin{\left(n x \right)} + 4 \cos{\left(4 x \right)} \cos{\left(n x \right)}$$