Function sine(x As Double, Optional term, Optional p = 1#) As Double
        If IsMissing(term) Then
            term = x
        End If
        
        If term <> 0 Then
            sine = term + sine(x, -term * x / (p + 1) * x / (p + 2), p + 2)
        End If
    End Function

Edit: This is my interpretation of the Taylor series approximation, but it's probably a bad idea to use this for anything that matters. One reason is that it doesn't reduce large angles, and another is that people who know what they're doing prefer to use some other formula, from a cursory Googling...though I'm not sure why.