Answer
cosh(-x) = cosh(x)
Work Step by Step
Use the definition of the hyperbolic cosine. Then, find $\cosh{(-x)}$ and re-arrange to arrive at $\cosh x$
$$cosh(x) = \frac{e^{x} + e^{-x}}{2}$$ cosh(-x) = $\frac{e^{-x} + e^{-(-x)}}{2}$ = $\frac{e^{-x} + e^{x}}{2}$ = cosh(x)
