Try this, but be sure that the Value of textbox1 is between -1 and 1:
TextBox2 = Application.WorksheetFunction.Acos(Val(TextBox1))
If you want to result in degrees, multiply it by 180/pi.
TextBox3 = Application.WorksheetFunction.Acos(Val(TextBox1)) * (180 / WorksheetFunction.Pi())
This might do the trick.
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