成功啦，大家可以参考我的
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<meta name="viewport" content="width=device-width,initial-scale=1,max-scale=1,min-scale=1">
<script type="text/javascript">
function count(){
var a=document.getElementById("txt1").value;
var b=document.getElementById("txt2").value;
var c=document.getElementById("select").value;
var sum;
switch(c){
case "+":sum=parseInt(a)+parseInt(b);
break;
case "-":sum=a-b;
break;
case "*":sum=a*b;
break;
case "/":sum=a/b;
break;
}
document.getElementById("fruit").value=sum;
}
</script>
</head>
<body>
<input type='text' id='txt1' />
<select id='select'>
<option value='+'>+</option>
<option value="-">-</option>
<option value="*">*</option>
<option value="/">/</option>
</select>
<input type='text' id='txt2' />
<input type='button' value=' = ' onclick="count()"/>
<input type='text' id='fruit' />
</body>
</html>
