# Coxeter- Exercise 2.2.1(ii)

- Author:
- AndrewLyon

- Topic:
- Geometry, Straight Lines, Quadrilaterals

Prove the following theorem:
(ii) The three diagonal lines of a complete quadrilateral are never concurrent. (They are naturally said to form the

*diagonal triangle*of the quadrilateral.) The quadrilateral*pqrs*has diagonal lines*a,b,*and*c*. The lines cannot be concurrent without the lines of the quadrilateral being concurrent which they cannot be by definition of a quadrilateral.## New Resources

## Discover Resources

Download our apps here: