Show that `f(x)=cos(2x+pi/4)` is an increasing function on `(3pi//8,7pi//8)dot`

Explanation

To show that the function \( f(x) = \cos(2x + \frac{\pi}{4}) \) is an increasing function on the interval \( \left( \frac{3\pi}{8}, \frac{7\pi}{8} \right) \), we will follow these steps:

### Step 1: Find the derivative of the function
We start by differentiating the function \( f(x) \):

\[ f'(x) = \frac{d}{dx} \left( \cos(2x + \frac{\pi}{4}) \right) \]
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