I've looked in a couple of books, and can't find this anywhere, i'm looking for the opposite of the cos funtion, i.e. I know the cos of the angle, and need to know the actual size of the angle, is there an implicit function? If not, how can I achieve the same result? Any help would be much appreciated...

## All 4 Replies

try this function :

``````Public Function ArcCos(ByVal X As Double) As Double
If X <> 1 Then
ArcCos = Atn(-X / Sqr(-X * X + 1)) + 2 * Atn(1)
Else
ArcCos = 0
End If
End Function``````

Thanks, i'll check if it works, how does it work?What's Atn?What's the theory behind it?Any help would be much appreciated, i'd like to understand you see...

Atn is arctangent

tan(x) = sin(x) / cos(x)
cos(x) = sqr(1 - sin(x)^2)
tan(x) = sin(x) / sqr(1 - sin(x)^2)

so we got ArcSin = X / sqr(-X * X + 1)
and ArcCos = 90 - ArcSin
so ArcCos = (-X / sqr(-X * X + 1)) + 2 * Atn(1)

Thankyou very much for quenching my thirst for knowledge!

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