theorem :: EUCLID_8:78

for p1, p2, q1, q2 being Element of REAL 3 holds |((p1 - p2),(q1 - q2))| = ((|(p1,q1)| - |(p1,q2)|) - |(p2,q1)|) + |(p2,q2)| by RVSUM_1:137;