90° + 60° + 30° = 180°. By using this formula, we can verify the angle sum property as well.
This portion is called the interior of the angle.
It works for this triangle. Since the interior angles add up to 180°, every angle must be less than 180°. In euclidean geometry, an angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle. (определение interior angle в cambridge academic content dictionary © cambridge university press).