<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>菜鸟学堂(edu.jb51.net)</title>
</head>
<body>
<p id="demo">单击按钮显示90度角的正切值。</p>
<button onclick="myFunction()">点我</button>
<script>
function myFunction(){
document.getElementById("demo").innerHTML=Math.tan(90);
}
</script>
</body>
</html>