theorem
  t*z divides (t+z)|^(2*n) - (t-z)|^(2*n)
  proof
    (-z)|^(2*n) = z|^(2*n) by POWER:1; then
    (-z)|^(2*n) - z|^(2*n) = 0; then
    t*z divides (t+z)|^(2*n) - (t-z)|^(2*n) + 0 by Th20;
    hence thesis;
  end;
