theorem Th23:
  Sum (((1/2) GeoSeq)^\(n+1)) = (1/2)|^n
proof
  set r = 1/2;
  thus Sum (((1/2) GeoSeq)^\(n+1)) = (r|^(n+1)) / (1-r) by Th22
    .= (1/2)|^n*r/r by NEWTON:6
    .= r|^n;
end;
