PHP comment écrir ma formule mathématique ?

<?php
function factorielle($valeur){
	$fact = 1;
	for($i = 1; $i <= $valeur; $i++)
		$fact *= $i;
	return $fact;
}

function combinaison($k, $n){
	return (factorielle($n) / (factorielle($k) * factorielle($n - $k)));
}

function maFonction($a, $b, $c){
	$resu = 1;
	for($k = $c; $k <= $b; $k++){
		$temp = combinaison($k, $b);
		$temp *= pow((1/$a), $k);
		$temp *= pow((1 - 1/$a), $b - $k);
		
		$resu *= $temp;
	}
	return $resu;
}
?>

<form action="#" method="post">
<?php
	if(empty($_POST))
	{?>
	a = <input type="text" name="a" /><br />
	b = <input type="text" name="b" /><br />
	c = <input type="text" name="c" /><br />
	<input type="submit" />
<?php
	}
	else
	{
	?>
	resultat = <input type="text" value="<?php echo maFonction($_POST['a'], $_POST['b'], $_POST['c']); ?>" /><br />
<?php } ?>
</form>