There are two ways to use the Acos trig function:

Application.WorksheetFunction.Acos(argument)
and
Application.Acos(argument)

I can not get the first on to work at all. The second only intermittenly.

RA(k) = Application.Acos(u(k))works only partially even though it is between
the range -1<=u(k))<=1.

When I add "180/pi*" in front of it gives me a type mismatch error?