function tab(a) {
    var aa1 = document.getElementById("td1");
    var aa2 = document.getElementById("td2");
    var bb1 = document.getElementById("div1");
    var bb2 = document.getElementById("div2");
    if (a == 1) {
        aa1.style.background = "url(pic/bg5.gif)";
        aa2.style.background = "url(pic/bg6.gif)";
        aa2.style.color = "black";
        bb1.style.display = "block";
        bb2.style.display = "none";
    }
    if (a == 2) {
        aa2.style.background = "url(pic/bg5.gif)";
        aa1.style.background = "url(pic/bg6.gif)";
        aa1.style.color = "black";
        bb1.style.display = "none";
        bb2.style.display = "block";
    }
}