<!DOCTYPE html>
<html>
<body>
<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>
<p id="demo"></p>
<script>
var a = 7;
var b = 8;
var x = a + b;
document.getElementById("demo").innerHTML = x;
</script>
</body>
</html>