其它
$\frac{7}{9}$, $\frac{5}{4}$  Subtract the sum from the difference of the above two fractions. 
Overseas Competition->Knowledge Point->Counting Modules->Inclusion-Exclusion Principle
Sum $=\frac{5}{4}+\frac{7}{9}=\frac{73}{36}$  Difference $=\frac{5}{4}-\frac{7}{9}=\frac{17}{36}$  Difference (Sum and difference) $=\frac{73}{36}-\frac{17}{36}=\frac{56}{36}=\frac{14}{9}=1\frac{5}{9}$ 