Как вариант...
<!DOCTYPE html>
<html>
<head>
<!--
<script src="http://code.jquery.com/jquery-latest.js"></script>
-->
<style type="text/css">
</style>
<script type="text/javascript">
function obrahunok(){
var a,b,rezultat;
a=document.getElementById('chis1').value;
b=document.getElementById('chis2').value;
rezultat=parseInt(a)+parseInt(b);
document.getElementById('chis3').value=rezultat;
}
</script>
</head>
<body>
<form id="dodavanya" name="dodavanya">
<label> vvedit chislo 1<input type="text" name="chis1" id="chis1"></label>
<label> vvedit chislo 2<input type="text" name="chis2" id="chis2"></label>
<input type="button" name="obrah" value="Обрахувати" onClick="obrahunok()">
<label> suma<input type="text" name="chis3" id="chis3"></label>
</form>
</body>
</html>