reserve x, y, z, s for ExtReal;
reserve i, j for Integer;
reserve n, m for Nat;

theorem Th9:
  y <> -infty & y <> +infty & x <> 0 & y <> 0 implies x / y <> 0
  proof
    assume that
A1: y <> -infty & y <> +infty and
A2: x <> 0 and
A3: y <> 0;
    assume x / y = 0;
    then y" = 0 by A2;
    hence thesis by A1,A3,XXREAL_3:82;
  end;
