public function f($x)
    {
        if (!is_numeric($x)) {
            throw new \InvalidArgumentException('x variable must be numeric value.');
        }

        $float_fx = exp(-0.5 * pow(($x - $this->float_mu) / $this->float_sigma, 2)) / ($this->float_sigma * sqrt(2 * pi()));

        if ($this->int_precision) {
            return round($float_fx, $this->int_precision);
        }

        return $float_fx;
    }