function quantprest(){
 	vatual=(document.calculadora.x1.value);
	vprest=(document.calculadora.x2.value);
	taxa=(document.calculadora.x3.value);

    vatual=vatual.toString().replace('.','');
    vprest=vprest.toString().replace('.','');
    taxa=taxa.toString().replace('.','');

    vatual=vatual.toString().replace(',','.');
    vprest=vprest.toString().replace(',','.');
    taxa=taxa.toString().replace(',','.');

 	vatual=eval(vatual);
	vprest=eval(vprest);
	taxa=eval(taxa);

	nprest = Math.abs((100*(Math.log(1/1-(vatual*(taxa/100)/vprest))/Math.log(1+(taxa/100))))/100);
	document.calculadora.qprest.value = nprest.toFixed(4)
}