The value of $\displaystyle arccosh$ is given as

$\displaystyle \ln {\color{red}(} x\pm\sqrt{x^2-1}{\color{red})} $,

but my book states that

$\displaystyle \ln {\color{red}(} x + \sqrt{x^2-1} {\color{red})} $

gives you the principle value. Is the principle value the positive one, the one we would normally take? I'm guessing you get this option due to the graph of $\displaystyle \cosh{x}$ being symmetrical about the y axis?

Thanks for clearing this one up.