# Coxeter- Exercise 2.5.1

- Author:
- Pinyi Yao

- Topic:
- Geometry

According to the figure, try to look for any line with four points on them. We can see that line e has points Q,G,P,F on it.P and Q are diagonal points for a new quadrangle EDHS. The harmonic set is H(QP,GF). Also, line f has points R,G,S,C on it. R and S are diagonal points for a new quadrangle PJIK. The harmonic set is H(RS,GC).