Chapter 5 - Trigonometric Identities - Section 5.1 Fundamental Identities - 5.1 Exercises - Page 196: 90

The answer to Exercise 89 is exactly the same us $y=\cos{4x}$.

From Exercise 89, we got $y=\cos{(-4x)} = \cos{4x}$, which means that $y=\cos{(-4x)}$ is equivalent to $y=\cos{4x}$ Therefore, the answer in Exercise 89 is exactly the same as $y=\cos{4x}$.