**Exercise 3.2**
:
Let

*D* be the point at infinity on the projective line, and let

*A*,

*B*,

*C* be three finite points. Show that

**Exercise 3.3**
If

*MN* is the line between two points

*M* and

*N* in the projective
plane, use the

parameterization and the results of
section

2.2.1 to show that the cross ratio of four points

*A*,

*B*,

*C*,

*D* on the line is