function show_sub(obj,num,total){
	var obj_id = document.getElementById('mainMenu');
	var obj_tag = obj_id.getElementsByTagName('dt');	
	for(var i=1; i<=total; i++){
		if(i == num){
			document.getElementById(obj+i).style.display = 'block';
			obj_tag[i-1].firstChild.className = 'on';	
		}else{
			document.getElementById(obj+i).style.display = 'none';
			obj_tag[i-1].firstChild.className = '';	
		}
	}
}