网吧会员折扣如何计算?

<script language='javascript'>
function isnull(str)
{
var nullpatn=/^\s*$/
return nullpatn.test(str)
}
function comput()
{
var num1=document.getElementById('num1').value
var num2=document.getElementById('num2').value
var num3=document.getElementById('num3').value
if(!isnull(num1) && !isnull(num2) && !isnull(num3))
{
num1=parseFloat(document.getElementById('num1').value)
num2=parseFloat(document.getElementById('num2').value)
num3=parseFloat(document.getElementById('num3').value)
document.getElementById('num4').value=formatnum(num2/((num3+num2)/num1))
document.getElementById('num5').value=formatnum(parseFloat(num2/((num3+num2)/num1))/num1*100)+"%"
}
}
</script>
<script language="vbscript">
function formatnum(num)
formatnum=formatnumber(num,2,-1)<