hola gracias por tu respuesta yo se que cuando es variable globlal se colocan despues de abrir el script pero si las saco y darle suma me dice NaN
<html>
<head>
<script languaje="javaScript" type="text/javascript">
//variables globales
var n1 = parseInt(document.getElementById("num1").value);
var n2 = parseInt(document.getElementById("num2").value);
var nav4= window.Event ? true : false;
function num(e)
{
var key = nav4 ? e.which : e.keyCode;
return (key<=13 ||(key>=47 && key <= 57));
}
function suma(){
resultado = n1+n2;
(document.getElementById("num3").value) = resultado;
}
function restar()
{
var n1 = parseInt(document.getElementById("num1").value);
var n2 = parseInt(document.getElementById("num2").value);
resultado = n1-n2;
(document.getElementById("num3").value) = resultado;
}
</script>
</head>
<body>
<form name="form1">
<table border="1" width="600" bgcolor="celeste">
<tr>
<td>
<table border="1" width="250">
<tr><td width="100">numero 1</td>
<td><input type="text" id="num1" onkeyPress="return num(event)"></td></tr>
<tr><td>numero 2</td>
<td><input type="text" id="num2" onkeyPress="return num(event)"></td></tr>
<tr><td>resultado</td>
<td><input type="text" id="num3" onkeyPress="return num(event)"></td></tr>
<tr><td colspan="2"><center>
<input type="button" value="Sumar" onclick="suma()"><input type="button" value="Restar" onclick="restar()"><input type="reset" value="Nuevo"></td></tr></td></tr>
<table></td>
</table>
</form>
</body>
</html>