Intanto facciamo questo:
codice:
<html>
<head>

<script language="JavaScript"> 
<!-- 

function Somma() {
	
	mainForm.Rfield.value = parseInt(mainForm.Afield.value) + parseInt(mainForm.Bfield.value);
   };


function Differenza() {
	
	mainForm.Rfield.value = parseInt(mainForm.Afield.value) - parseInt(mainForm.Bfield.value);
   };

// -->
</script>

</head>
<body>
<tt>
 <form name="mainForm">

  Campo A = <input type="text" name="Afield" size="20" value="">

  Campo B = <input type="text" name="Bfield" size="20" value="">


  

  Campo R = <input type="text" name="Rfield" size="20" value="" readonly> ( Risultato )

  

  <input value="somma  A + B"       type="button" onClick="Somma()"     >

  <input value=" sottrai   A  -  B" type="button" onClick="Differenza()">

 </form>
</tt>

</body>
</html>