请大神看看哪里错了，提交显示空白
<!DOCTYPE HTML>
<html >
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>函数</title>
<script type="text/javascript">
function num(a,b){
if(a>b){
return:a
}else if(a<b){
return:b
}else
return:a||b;
};
document.write(" 5 和 4 的较大值是:"+"num(5,4)"+"<br>");
document.write(" 6 和 3 的较大值是:"+"num(6,3)" );
</script>
</head>
<body>
</body>
</html>
