function calculate() {
 var nInvoice = parseFloat(document.loan.invoice.value);
 var nDown = parseFloat(document.loan.down.value);
 var nN= document.loan.length.selectedIndex;
 var nTerm = parseFloat(document.loan.length[nN].value);
 var nRate = parseFloat(document.loan.rate.value); 
 var nTrade = parseFloat(document.loan.trade.value);
 
 var nFinanced =  (nInvoice ) - (nTrade + nDown);

  var nI = nRate / 100 / 12;

  var nY = Math.pow(1 + nI, nTerm);

 
  var nPayment = (nFinanced * nY * nI) / (nY -1);
  var nInterest  = (nPayment * nTerm) - nFinanced;
  
  
  document.loan.interest.value =Math.round(nInterest);
  document.loan.payment.value=Math.round(nPayment);
  document.loan.financed.value=Math.round(nFinanced);
 }