Evaluating An Exponential Function

Algebra Level 3

$\large f(x)=\frac{e^x+e^{-x}}{e^x-e^{-x}}$

Suppose we define the function $f(x)$ as above. If $f(a)=\frac{5}{3}$ and $f(b)=\frac{7}{5},$ what is the value of $f(a+b)?$