//Payment Calculator v2

               
function showpay() {
 if ((document.calc.price.value == null || document.calc.price.value.length == 0) || 
     (document.calc.months.value == null || document.calc.months.value.length == 0) ||
     (document.calc.rate.value == null || document.calc.rate.value.length == 0) || 
     (document.calc.down.value == null || document.calc.down.value.length == 0) || 
     (document.calc.trade.value == null || document.calc.trade.value.length == 0))
 { document.calc.pay.value = "Incomplete data";
 }
 else
 {
 
 var price = document.calc.price.value;
 var down = document.calc.down.value;
 var trade = document.calc.trade.value;
 var princ = (price-down)-trade;
 var term  = document.calc.months.value;
 var intr = document.calc.rate.value;
 
 if(intr != 0) {
  intr = intr/1200;
  document.calc.pay.value = roundNumber(princ * intr / (1 - (Math.pow(1/(1 + intr), term))), 2);
 }
 else {
  document.calc.pay.value = roundNumber(princ / term, 2);
 }
 
 
 }

// payment = principle * monthly interest/(1 - (1/(1+MonthlyInterest)*Months))

}

function roundNumber(num, dec) {
  var result = Math.round(num*Math.pow(10,dec))/Math.pow(10,dec);
  return result;
}