Chapter 5 - Trigonometric Identities - Section 5.1 Fundamental Identities - 5.1 Exercises - Page 194: 20

$\cos\theta=3$ is wrong. The domain of $\cos\theta$ only ranges from $-1$ to $1$.

The problem here is with the statement $\cos\theta=3$. As we know, the domain of $\cos\theta$ only ranges from $-1$ to $1$. In other words, the value of $\cos\theta$ cannot exceed $1$. Therefore, $\cos\theta=3$ is impossible.