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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Fmpq教程(fmpq.com)</title>
</head>
<body>
<p id="demo">单击按钮显示0.5的反余弦值。</p>
<button onclick="myFunction()">点我</button>
<script>
function myFunction(){
    document.getElementById("demo").innerHTML=Math.acos(0.5);
}
</script>
</body>
</html>