I think it has sometihng to do with the graph of cosh always being positive.
Therefore only is the only one which is always positive.
The value of is given as
,
but my book states that
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 being symmetrical about the y axis?
Thanks for clearing this one up.
A justification of this is required. A justification can be found here: http://www.math.cornell.edu/~kbrown/122/coshinv.pdf