Flash Math "Taux de rendement" forumula? Se confondre!

Question

Voici ma formule, Ne pas obtenir le bon rendement .... Je cherche le paiement mensuel que quelqu'un devrait contribuer (avec un taux de rendement%) pour obtenir leur "valeur future" !! Je fais quelque chose de mal .. Je crois que c'est vraiment simple .. Merci.

//set default input text 
var loanText = 100000; 
var interestText = 10; 
var yearsText = 1; 

//restrict the input boxes to numbers and symbols 
loan_txt.restrict = "0-9\\."; 
interest_txt.restrict = "0-9\\."; 
years_txt.restrict = "0-9\\."; 
// 
calculate_mc.onRollOver = function(){ 
    this.gotoAndPlay("over"); 
} 
calculate_mc.onRollOut = function(){ 
    this.gotoAndPlay("out"); 
} 
calculate_mc.onRelease = function(){ 
    if (loan_txt.text!="" && interest_txt.text!="" && years_txt.text!=""){ 
     loanPayments(); 
    }else{ 
     monthly_txt.text = "Please fill in all fields"; 
    } 
} 
// 
loan_txt.onSetFocus = function() { 
    if (loan_txt.text == loanText) { 
     loan_txt.text = ""; 
    } 
}; 
loan_txt.onKillFocus = function() { 
    if (loan_txt.text == "") { 
     loan_txt.text = loanText; 
    } 
}; 
interest_txt.onSetFocus = function() { 
    if (interest_txt.text == interestText) { 
     interest_txt.text = ""; 
    } 
}; 
interest_txt.onKillFocus = function() { 
    if (interest_txt.text == "") { 
     interest_txt.text = interestText; 
    } 
}; 
years_txt.onSetFocus = function() { 
    if (years_txt.text == yearsText) { 
     years_txt.text = ""; 
    } 
}; 
years_txt.onKillFocus = function() { 
    if (years_txt.text == "") { 
     years_txt.text = yearsText; 
    } 
}; 
loanPayments = function(){ 
    var loanAmount = loan_txt.text; 
    var interestRate = (interest_txt.text)/100; 
    var years = years_txt.text; 
    var downPayment = 0; 

    var monthRate = interestRate/12; 
    var numPayments = years*12; 
    var prin = loanAmount - downPayment; 

    monthPayment = Math.floor((prin*monthRate)/(1-Math.pow((1+monthRate),(1*numPayments)))*100)/100; 
       //form.NumberOfPayments.value=NumPayments 
    var getDec = String(monthPayment).indexOf("."); 
    var add0s = ""; 
    if(String(monthPayment).length +getDec == 2){ 
     add0s = "0"; 
    }else if(String(monthPayment).length +getDec == 1){ 
     add0s = "00"; 
    } 
    trace(add0s); 
    trace(monthPayment); 
    monthly_txt.text = "$"+monthPayment+add0s; 
} 

Answer 1

monthPayment = (prin*monthRate)/(1-Math.pow(1+monthRate,-numPayments))