I am studying now calculus and I am learning about hyperbolic function. But I can't understand why hyperbolic functions are called 'hyperbolics' Is there any relation between hyperbolic function and hyperbola? And why there are natural constant e in function? I saw some information that notices the relation between them in Wikipedia, so I try to find the relation as integrating the hyperbola, but I can't solve that. Please tell me why they are called 'hyperbolic' and derivate the hyperbolic function. (If the hyperbolic function is just defined so, please tell me why they are defined so.)
I don't really get why these hyperbolic functions are named after trig functions. Can someone enlighten me?
I have a directory that contains the following:  x.pdf y.zip z.mp3 a.pdf   I want to delete all files apart from x.pdf and a.pdf. How do I do this from the terminal? There are no subdirectories so no need for any recursion.