triangle in which one angle is a 90-degree angle
A right triangle, (also called a right-angled triangle), has one angle that is 90 degrees. The other two angles always add up to 90 degrees but can be different sizes. The side opposite to the right angle is the hypotenuse; it is the longest side in the right triangle. The other two sides are the legs or catheti (singular: cathetus) of the triangle.
Missing sides or angles of a right triangle can be found using trigonometry, the Pythagorean theorem, and many other ways. In any triangle all the angles add up to 180 degrees.