reserve a, b, c, d, e for Complex;

theorem Th120: :: REAL_1'40_2
  a / c - b / c = (a - b) / c
proof
  thus a/c - b/c = a/c+ -b/c .=a/c+(-b)/c by Lm17
    .=(a+ -b)/c by Th62
    .=(a-b)/c;
end;
