//this function change the background of the btn on mouseover
function changeActiveImage(btn_num){
var leftmenu=document.getElementById("left_menu").getElementsByTagName("li")[btn_num];
leftmenu.className="active";
}

//this function change the background of the btn on mouseout
function changeNoActiveImage(btn_num){
var leftmenu=document.getElementById("left_menu").getElementsByTagName("li")[btn_num];
leftmenu.className="noactive";
}