reserve i,j for Nat;

theorem Th38:
  for K being Ring,M1,M2 being Matrix of K st len M1=len M2 &
  width M1=width M2 holds M1=M1+M2+(-M2)
proof
  let K be Ring,M1,M2 be Matrix of K;
A1: M1+M2+(-M2)=M1+M2-M2;
  assume len M1=len M2 & width M1=width M2;
  hence thesis by A1,Th21;
end;
