Chapter 7 - Exponential Functions - 7.9 Hyperbolic Functions - Exercises - Page 384: 5

See the details below.

We refer to Figure 2 on page 377. We can see that since $\sinh x$ is an odd function, then for each $t$, $\sinh x=t$ has only one solution. But since $\cosh x$ is a positive even function, then for each $t\gt 1$, $\cosh x=t$ has two solutions.