acos (arc cosine) is one of several extended math command available in Tclx.
DGP Tclx?!?! What? The acos function has been supported by plain old expr in plain old Tcl since at least release 7.4.
It is a procedure which provides command access to the expr math function, taking the same arguments as the expr function and taking expressions as arguments as well.
DGP This sounds like more description of some ridiculously out of date thing from TclX. If we must still talk about such matters, let's please shove them over in a corner somewhere, and leave this page for discussing the current, standard, in plain old Tcl acos function.
LV 2007 Oct 31 Umm - the original text of this page was written in 2002 - no one came along and updated it until today. So just update the contents - no need to get emotional over it...
One good use for acos is to get the constant Pi:
% expr acos(-1) 3.141592653589793 % tcl::mathfunc::acos -1 ;# In Tcl 8.5 3.141592653589793