JS案例：通过常量PI求圆的周长
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<meta charset="utf-8">
<title>求圆的周长案例示例</title>
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<script type="text/javascript">
var r=2.5; //声明圆的半径r
const p=Math.PI;
var c=p*2*r; //声明圆的周长并求出其值
document.write("半径为2.5的圆的周长为："+ c);
</script>
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