sqrtの課題

/**
 * Copyright Nowloading_ ( http://wonderfl.net/user/Nowloading_ )
 * MIT License ( http://www.opensource.org/licenses/mit-license.php )
 * Downloaded from: http://wonderfl.net/c/fT5X
 */

package {
    import flash.display.Sprite;
    import flash.events.MouseEvent;
    import flash.text.TextField;
    
    public class FlashTest extends Sprite {
        private var spr:Sprite;
        private var tf:TextField;
        public function FlashTest() {
            init();
        }
        private function init():void{
            spr = new Sprite();
            addChild(spr);
            spr.graphics.beginFill(0x000000);
            spr.graphics.drawRect(-2,-2,4,4);
            spr.graphics.endFill();
            spr.x = stage.stageWidth/2;
            spr.y = stage.stageHeight/2;
            
            tf = new TextField();
            addChild(tf);
            stage.addEventListener(MouseEvent.MOUSE_MOVE,onMouseMove);            
        }
        public function onMouseMove(e:MouseEvent):void{
            graphics.clear();
            graphics.lineStyle(1,0,1);
            graphics.moveTo(spr.x,spr.y);
            graphics.lineTo(mouseX,mouseY);
            
            var dx:Number = spr.x - mouseX;
            var dy:Number = spr.y - mouseY;
            var dist:Number =Math.sqrt(dx*dx +dy*dy);
            tf.text = dist.toString();
        }


    }
}