VML 缩放后坐标的偏移量

[okbaozhen]

mainGroup.attachEvent("onmousewheel" , mouseWheel , false);

function mouseWheel(){
		var delta = 0;  		
		if(event.wheelDelta) delta = event.wheelDelta/120;
		alert(delta);
		if(delta){
			taskH+=delta;
			if(taskH<-5) taskH = -5;
			else if (taskH > 5) taskH =5;
			//mainGroup.coordsize = xx/(1+0.1*taskH) + "," + yy/(1+0.1*taskH);
			mainGroup.style.zoom=1+0.1*taskH;
		}
}

比如初始有一点P（X，Y），缩放一次后该P点的坐标怎么计算？？？求解答！！！