求助，找不到问题在哪里
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var txt1=document.getElementByid("txt1").value;
var txt2=document.getElementByid("txt2").value;
var select=document.getElementByid("select").value;
var sum;
switch(select){
case "+":
sum=txt1+txt2;
break;
case "-":
sum=txt1-txt2;
break;
case "*":
sum=txt1*txt2;
break;
case "/":
sum=txt1/txt2;
}
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()"/> <!--通过 = 按钮来调用创建的函数，得到结果-->
<input type='text' id='fruit' />
</body>
</html>
