<form name="form1">
<input name=a value=$a onchange="form1.c.value=form1.a.value+form1.b.value;">
<input name=b value=$b onchange="form1.c.value=form1.a.value+form1.b.value;">
<input name=c>
</form>
<input name=a value=$a onchange="form1.c.value=form1.a.value+form1.b.value;">
<input name=b value=$b onchange="form1.c.value=form1.a.value+form1.b.value;">
<input name=c>
</form>
<form name="form" method="post" action="">
<input name=a value=1 onchange=sum()>
<input name=b value=2 onchange=sum()>
<input name=c >
</form><script language="JavaScript">
function sum()
{
form.c.value=parseInt(form.a.value)+parseInt(form.b.value);
}
</script>