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12.17.2  The cross ratio of four collinear points: cross_ratio

The cross_ratio command takes as arguments four complex numbers a,b,c,d.

cross_ratio returns the cross ratio [(ca)/(cb)]/[(da)/(db)].
Input:

cross_ratio(0,1,2,3)

Output:

4/3

Input:

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

Output:

-1

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