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<!DOCTYPE html>
<html>
<body>
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<p>典型的算术运算会操作两个数（或表达式）并返回一个新的数。</p>
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<p id="demo"></p>
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<script>
var a = 2;
var x = (7 + 8) * a;
document.getElementById("demo").innerHTML = x;
</script>
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</body>
</html>
​