为什么只输出5、8、3和16，没有7、1、4和12啊
<!DOCTYPE HTML>
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<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>函数传参</title>
<script type="text/JavaScript">
function sum(x,y,z)
{
sum = x + y +z;
document.write(x+"、"+y+"、"+z+"和:"+sum+"<br/>");
}
sum(5,8,3);
sum(7,1,4);
</script>
</head>
<body>
</body>
</html>
