theorem RI2:
  a1|^(n+k) - b1|^(n+k) = a1|^n*(a1|^k - b1|^k) +
  b1|^k*(a1|^n - b1|^n)
  proof
    a1|^(n+k) = a1|^n * a1|^k & b1|^(n+k) = b1|^n*b1|^k by NEWTON:8;
    hence thesis;
  end;
