求助，照着《javascript权威指南》上写的小例子，就是不行

用javascript编写的借贷支付金额计算器
各位帮我调试看看，明明没错误 ，输入所以数据后没反应
<html>
<head>
<title>Javascript loan Calculator</title>
<style>
.result{font-weight:bold;}
#payment{text-decoration:underline;}
</style>
</head>
<body>
<form name="loandata">
<table>
  <tr><td><b>Enter loan Information :</b></td></tr>
  <tr>
   <td>1)Amount of the loan (any currency):</td>
    <td><input type="text" name="principal" onChange="calculate();"></td>
   </tr>
   <tr>
     <td>2)Annual percentage rate of interest:</td>
     <td><input type="text" name="interest" onChange="calculate();"></td>
   </tr> 
<tr>
<td>3) Repayment period in years:</td>
<td><input type="text" name="years" onChange="calculate();"></td>
</tr>
<tr><td></td>
<td><input type="button" value="Compute" onClick="calculate();"></td>
</tr>
<tr><td><b>Payment Information :</b></td></tr>
<tr>
<td>4)Your monthly payment:</td>
<td>$<span class="result" id="payment"></span></td>
</tr>
<tr>
<td>5)Your total payment:</td>
<td>$<span class="result" id="total"></span></td>
</tr>
<tr>
<td>6)Your total interest payments:</td>
<td>$<span class="result" id="totalinterest"></span></td>
</tr>
</table>
</form><script type="text/JavaScript">
function calculate(){ var principal=document.loandata.principal.value;
var interest=document.loandata.interest.value/100/12;
var payments=document.loandata.years.value*12; var x=Math.pow(1+interest,payments);
var monthly=(principal*x*interest)/(x-1);

var payment=document.getElementById("payment");
var total=document.getElementBuId("total");
var totalinterest=document.getElementById("totalinterest");
document.write("hello word"); if(isFinite(monthly)){
document.write("hi");
payment.innerHTML=monthly.toFixed(2);
total.innerHTML=(monthly*payments).toFixed(2);
totalinterest.innerHTML=((monthly*payments)-principal).toFixed(2);
}else{
  document.write("hello");
payment.innerHTML="";
total.innerHTML="";
totalinterest.innerHTML="";
}
}
</script>
</body>
</html>