### 26.16.2 Cross ratio of four collinear points

The cross ratio of four numbers a,b,c,d is [(c−a)/(c−b)]/[(d−a)/(d−b)].

The cross_ratio command finds the cross ratio.

- cross_ratio takes four arguments: a,b,c,d, complex numbers.
- cross_ratio(a,b,c,d) returns the cross ratio, [(c−a)/(c−b)]/[(d−a)/(d−b)].

#### Examples

cross_ratio(0,1,2,3)

4
3

cross_ratio(i,2+i,3/2+i, 3+i)

−1 |  |  |  |  |  |  |  |  |  |