代码
<html>
<script>
function myFunction() {
a=document.getElementById("a").value;
op=document.getElementById("op").value;
b=document.getElementById("b").value;
if(op=="+")c=a+b;
if(op=="-")c=a-b;
if(op=="*")c=a*b;
if(op=="/")c=a/b;
document.getElementById("demo").innerHTML = c;
}
</script>
<body>
<input type="number" id="a">
<select id="op">
<option value="+">+</option>
<option value="-">-</option>
<option value="*">*</option>
<option value="/">/</option>
</select>
<input type="number" id="b">
<input type="submit" onclick="myFunction()">
<p id="demo">?</p>
</body>
</html>运行：
