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                        /JavaScript進階篇

求助，找不到問題在哪里

<!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()"/> 
?? <input type='text' id='fruit' />??
?</body>
</html>