# The cross ratio of four collinear points: cross_ratio

### 13.17.2 The cross ratio of four collinear points: cross_ratio

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 for 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.

- Input:
  cross_ratio(0,1,2,3)
  Output:

  4
  3

- Input:
  cross_ratio(i,2+i,3/2 + i, 3+i)
  Output:

  −1