theorem Th27:
  m <> 0 & n <> 0 implies
  denominator((i/m)/(n/j)) = (m*n) div ( (i*j) gcd (m*n) ) &
  numerator((i/m)/(n/j)) = (i*j) div ( (i*j) gcd (m*n) )
  proof
    (i/m)/(n/j) = (i*j)/(m*n) by XCMPLX_1:84;
    hence thesis by Th15;
  end;
