theorem Th56:
  |.p.| = sqrt ((p.1)^2+(p.2)^2+(p.3)^2)
proof
  |.p.| = sqrt Sum<* p.1*p.1,p.2*p.2,p.3*p.3 *> by Th43
      .= sqrt ((p.1)^2+(p.2)^2+(p.3)^2) by RVSUM_1:78;
  hence thesis;
end;
