fp3 hypobolic functions,Watch this thread
Osborn's rule allows you to convert a trigonometric identity into an equivalent hyperbolic identity. It has nothing to do with differentiation!
Seriously, forget all about it! I think I saw Osborn's rule once when I first learnt hyperbolics and then never used it again - it's as easy to derive the identities directly as it is to remember the correct conditions for applying the rule