reserve s for set,
  i,j for natural Number,
  k for Nat,
  x,x1,x2,x3 for Real,
  r,r1,r2,r3,r4 for Real,
  F,F1,F2,F3 for real-valued FinSequence,
  R,R1,R2 for Element of i-tuples_on REAL;

theorem Th90:
  Sum(R1 - R2) = Sum R1 - Sum R2
proof
  thus Sum(R1 - R2)= Sum R1 + Sum -R2 by Th89
  .= Sum R1 +- Sum R2 by Th88
  .= Sum R1 - Sum R2;
end;
