let n be Nat; :: thesis: for p1, p2, p3 being Point of (TOP-REAL n) holds |((p1 + p2),p3)| = |(p1,p3)| + |(p2,p3)|
let p1, p2, p3 be Point of (TOP-REAL n); :: thesis: |((p1 + p2),p3)| = |(p1,p3)| + |(p2,p3)|
reconsider f1 = p1, f2 = p2, f3 = p3 as FinSequence of REAL by EUCLID:27;
reconsider q1 = p1, q2 = p2 as Element of REAL n by EUCLID:25;
len f2 = n by FINSEQ_1:def 18;
then A1: ( len f1 = len f2 & len f2 = len f3 ) by FINSEQ_1:def 18;
p1 + p2 = q1 + q2 by EUCLID:68;
hence |((p1 + p2),p3)| = |(p1,p3)| + |(p2,p3)| by A1, RVSUM_1:150; :: thesis: verum