Basic Trigonometry: flipping an image around a Y-axis. ??

[cliffdangler]

Hi,

I have this code that rotates an object (a square) around a y axis.

onClipEvent (load) {
	radius = 100;
	speed = 5;
	xcenter = 275;
	zcenter = 100;
	angle = 0;
	fl = 100;
}
onClipEvent (enterFrame) {
	z = Math.sin(angle*Math.PI/180)*radius+zcenter;
	scale = fl/(fl+z);
	_x = Math.cos(angle*Math.PI/180)*radius+xcenter;
	_xscale = _yscale = scale*100;
	angle += speed;
}

The problem is that I want the object to flip around as it goes around the radius (so that the user will see the reverse side of the square). I've tried using -Math.sin() on the _xscale but it's not perfect.

Please help a Math dropout. Thanks for your help.