reserve a,b,c,d for Real;
reserve r,s for Real;

theorem Th80:
  b < 0 & c <= a*b implies a <= c/b
proof
  assume that
A1: b<0 and
A2: a*b>=c;
  a*b/b<=c/b by A1,A2,Lm30;
  hence thesis by A1,XCMPLX_1:89;
end;
