Как-то так.
<html>
<head>
<title>Курс по Javascript</title>
<meta http-equiv="content-type" content="text/html; charset=utf8" />
</head>
<body>
<div>
<script type="text/javascript">
function kalk(){
var n1=document.getElementById('num1').value
var n2=document.getElementById('num2').value
n1=parseFloat(n1)
n2=parseFloat(n2)
var a=document.getElementById('act').value
var r=0
if(a==1) r=n1+n2
if(a==2) r=n1-n2
if(a==3) r=n1*n2
if(a==4){
if(n2==0){
alert("на 0 делить нельзя")
document.getElementById("num2").value=""
document.getElementById("num2").focus()
}
else r=n1/n2
}
if(a==5) r=n1/100*n2
if(a==6) r=Math.pow(n1,n2)
if(a==7) r=Math.pow(n1,0.5)
document.getElementById("result").value=r
}
function addToInput1(value) {
document.querySelector('#num1').value += value;
}
function addToInput2(value) {
document.querySelector('#num2').value += value;
}
function clean() {
document.querySelector('#num1').value = '';
document.querySelector('#num2').value = '';
document.querySelector('#result').value = '';
}
</script>
<form id="form1" name="form1" method="post" action="">
<table width="400" border="1" align="center" cellpadding="5" cellspacing="0">
<tr>
<td>Число 1</td>
<td><input type="text" name="num1" id="num1" /><br />
<input onclick="addToInput1(this.value)" type="button" name="rakam11" value="1" />
<input onclick="addToInput1(this.value)" type="button" name="rakam22" value="2" />
<input onclick="addToInput1(this.value)" type="button" name="rakam33" value="3" />
<input onclick="addToInput1(this.value)" type="button" name="rakam44" value="4" />
<input onclick="addToInput1(this.value)" type="button" name="rakam55" value="5" />
<input onclick="addToInput1(this.value)" type="button" name="rakam66" value="6" />
<input onclick="addToInput1(this.value)" type="button" name="rakam77" value="7" />
<input onclick="addToInput1(this.value)" type="button" name="rakam88" value="8" />
<input onclick="addToInput1(this.value)" type="button" name="rakam99" value="9" /></td>
</tr>
<tr>
<td>Число 2</td>
<td><input type="text" name="num2" id="num2" /><br />
<input onclick="addToInput2(this.value)" type="button" name="rakam1" value="1" />
<input onclick="addToInput2(this.value)" type="button" name="rakam2" value="2" />
<input onclick="addToInput2(this.value)" type="button" name="rakam3" value="3" />
<input onclick="addToInput2(this.value)" type="button" name="rakam4" value="4" />
<input onclick="addToInput2(this.value)" type="button" name="rakam5" value="5" />
<input onclick="addToInput2(this.value)" type="button" name="rakam6" value="6" />
<input onclick="addToInput2(this.value)" type="button" name="rakam7" value="7" />
<input onclick="addToInput2(this.value)" type="button" name="rakam8" value="8" />
<input onclick="addToInput2(this.value)" type="button" name="rakam9" value="9" />
</td>
</tr>
<tr>
<td>Действие</td>
<td><select name="act" id="act">
<option value="1">+</option>
<option value="2">-</option>
<option value="3">*</option>
<option value="4">/</option>
<option value="5">%</option>
<option value="6">степени</option>
<option value="7">Корень</option>
</select></td>
</tr>
<tr>
<td>Равно</td>
<td><input type="text" name="result" id="result" /></td>
</tr>
<tr>
<td colspan="2"><input type="button" name="button" id="button" onclick="kalk()" value="Считать" />
<input type="button" name="button1" onclick="clean()" value="CLEAN" /></td>
</tr>
</table>
</form>
<p> </p>
</body>
</html>