Thales' theorem states if a triangle inscribed in a circle has a diameter for one side, one of these things will be located opposite that diameter.
The Lambert quadrilateral is a quadrilateral which possesses three of these things, while a Saccheri quadrilateral has two of them. Euclid's fourth postulate states that all of these things are congruent. The area of a triangle in spherical geometry which has three of these things is equal to one-half pi times the radius of the sphere squared. Thales' theorem states if a triangle inscribed in a circle has a diameter for one side, one of these things will be located opposite that diameter. The Pythagorean theorem applies to triangles that have one of them. For 10 points, name these angles formed by the junction of two perpendicular lines and found in the corners of rectangles.
The Lambert quadrilateral is a quadrilateral which possesses three of these things, while a Saccheri quadrilateral has two of them.
right angles [or 90 degree angles]