Before learning the 90-degree angle formula, let us recall a few things about a 90-degree angle. When the angle between two rays is equal to 90 degrees, then the angle formed is called a right angle. Pythagoras theorem is used in a right-angled triangle. It states that the sum of the squares of lengths of the adjacent and opposite sides of a right triangle is equal to the square of the length of the hypotenuse side. Further, this 90-degree angle formula can be used to determine the right angle.