Cosine Function

I can't seem to get this function to work. Visual studio does not recognize cos(xxx) or sin(xxx).  I have tried adding a reference to system.math, but it does not appear in the add reference window.  What reference to do I need to add?

Also, once cos does work, how does one enter ther following properly?

x = cos^-1(y)
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Fernando SotoRetiredCommented:
It is in the System namespace

Math.Cos(xxx) where xxx is defined as a Double and returns a Double
Math.Sin(xxx) where xxx is defined as a Double and returns a Double
Fernando SotoRetiredCommented:
The formula x = cos^-1(y) converts to

Dim x As Double
Dim y As Double

x = Math.Acos(y)

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Visual Basic.NET

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