Hehe, I know what it does by the name, but can you tell more specifically what it does?
Tan = b/a, rite?
So ArcTan... ArcTan2...
SHARE YOUR WISDOM GOD DAMIT! 
Btw, I have a same like function, though how it works is simplier, but does the job though:
SCAR Code:
Function InPizzaSlice(xx, yy, StartR, EndR, MidX, MidY, MaxRadius: Integer): Boolean;
var
Rx, Ry, Dx, Dy, D: Integer;
Begin
Rx := Round( MaxRadius * Sine(StartR)) + MidX;
Ry := Round(- MaxRadius * Cose(StartR)) + MidY;
Dx := Round( MaxRadius * Sine(EndR)) + MidX;
Dy := Round(- MaxRadius * Cose(EndR)) + MidY;
If (XX > Rx) And (XX < Dx) Then
Begin
Result := (Distance(xx, yy, MidX, MidY) <= MaxRadius);
End;
End;

But hey if not post, or make a tut, can you wizzy tell me about those ArcTans etc and where, how can they be used for effectively, like in your function..