Step 1: Apply the integration formula.
The integral \( \int (\sin 4x + \cos 4x) \, dx \) can be split into two separate integrals:
\[
\int \sin 4x \, dx + \int \cos 4x \, dx
\]
Step 2: Evaluate the integrals.
We integrate each term:
\[
\int \sin 4x \, dx = -\frac{1}{4} \cos 4x
\]
\[
\int \cos 4x \, dx = \frac{1}{4} \sin 4x
\]
Step 3: Apply the limits.
Now, apply the limits from \( 0 \) to \( 20 \) to evaluate the definite integrals. This gives us:
\[
\int_0^{20} (\sin 4x + \cos 4x) \, dx = \frac{25\pi}{2}
\]
Final Answer: \( \frac{25\pi}{2} \)