public function dividir(Rational $r)
 {
     $num = $this->getNum() / $r->getNum();
     $den = $this->getDen() / $r->getDen();
     $nuevo = new Rational();
     $nuevo->setNum($this->getNum()) / $r->getNum();
     $nuevo->setDen($this->getDen() / $r->getDen());
 }