Derivative of exp(x)*cos(x/3)

1. Apply the product rule:

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

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

   1. The derivative of $e^{x}$ is itself.

   $g{\left(x \right)} = \cos{\left(\frac{x}{3} \right)}$; to find $\frac{d}{d x} g{\left(x \right)}$:

   1. Let $u = \frac{x}{3}$.

   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 $\frac{d}{d x} \frac{x}{3}$:

      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: $\frac{1}{3}$

      The result of the chain rule is:

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

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

2. Now simplify:

   $$\left(- \frac{\sin{\left(\frac{x}{3} \right)}}{3} + \cos{\left(\frac{x}{3} \right)}\right) e^{x}$$

The answer is:

$$\left(- \frac{\sin{\left(\frac{x}{3} \right)}}{3} + \cos{\left(\frac{x}{3} \right)}\right) e^{x}$$