The diagonals bisect at right angles. The sum of interior angles in a quadrilateral is 360°. This fact and the properties of quadrilaterals can be used to calculate angles.

Get a fresh, colourful angle on the basics of geometry! What are angles? Do you know the difference between a triangle and a quadrilateral? These videos on the basics of geometry will help you ...

The nine chapters deal respectively with preliminary ideas, trihedral angles, skew quadrilaterals, tetrahedra, transversals, oblique cones, spheres, inversion, and recent geometry of the tetrahedron.