derivative of `e^(mx).cos n x`

Answer

To find the derivative of the function \( f(x) = e^{mx} \cos(nx) \), we will use the product rule of differentiation. The product rule states that if you have two functions \( u(x) \) and \( v(x) \), then the derivative of their product is given by:

\[ \frac{d}{dx}(uv) = u \frac{dv}{dx} + v \frac{du}{dx} \]

### Step-by-Step Solution:

1. **Identify the Functions**:
Let:
- \( u = e^{mx} \)
- \( v = \cos(nx) \)

2. **Differentiate \( u \) and \( v \)**:
- The derivative of \( u \) with respect to \( x \):
\[ \frac{du}{dx} = \frac{d}{dx}(e^{mx}) = e^{mx} \cdot \frac{d}{dx}(mx) = e^{mx} \cdot m \]
- The derivative of \( v \) with respect to \( x \):
\[ \frac{dv}{dx} = \frac{d}{dx}(\cos(nx)) = -\sin(nx) \cdot \frac{d}{dx}(nx) = -\sin(nx) \cdot n \]

3. **Apply the Product Rule**:
Now, using the product rule:
\[ \frac{d}{dx}(e^{mx} \cos(nx)) = u \frac{dv}{dx} + v \frac{du}{dx} \]
Substitute \( u \), \( v \), \( \frac{du}{dx} \), and \( \frac{dv}{dx} \):
\[ = e^{mx} \cdot \left(-n \sin(nx)\right) + \cos(nx) \cdot (m e^{mx}) \]

4. **Simplify the Expression**:
Combine the terms:
\[ = -n e^{mx} \sin(nx) + m e^{mx} \cos(nx) \]

5. **Final Result**:
Thus, the derivative of \( f(x) = e^{mx} \cos(nx) \) is:
\[ f'(x) = e^{mx} \left( m \cos(nx) - n \sin(nx) \right) \]