public function tan(int $scale = null): Decimal
    {
	    $cos = $this->cos($scale + 2);
	    if ($cos->isZero()) {
	        throw new \DomainException(
	            "The tangent of this 'angle' is undefined."
	        );
	    }

	    return $this->sin($scale + 2)->div($cos)->round($scale);
    }