theorem :: EUCLID_8:77

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_1:136;