丸書くには三角関数を使わなきゃだめか・・・　

/**
 * Copyright kazuyuki ( http://wonderfl.net/user/kazuyuki )
 * MIT License ( http://www.opensource.org/licenses/mit-license.php )
 * Downloaded from: http://wonderfl.net/c/bb24
 */

package {
    import flash.display.Sprite;
    public class FlashTest extends Sprite {
        public function FlashTest() {
            var centerX:Number =0;
            var centerY:Number =100;
            var r:Number =50;
             
            graphics.lineStyle(1,0x888888);
            graphics.beginFill(0xff0000);
            graphics.moveTo(centerX,centerY);
            graphics.curveTo(0,0,100,0);
            graphics.curveTo(200,0,200,100);
            graphics.curveTo(200,200,100,200);
            graphics.curveTo(0,200,0,100);
            graphics.endFill();
         }
    }
}