а) $$ \frac{55}{99} $$ = $$ \frac{5 \cdot 11}{9 \cdot 11} $$ = $$ \frac{5}{9} $$,
$$ \frac{555}{999} $$ = $$ \frac{5 \cdot 111}{9 \cdot 111} $$ = $$ \frac{5}{9} $$.
б) $$ \frac{1313}{7777} $$ = $$ \frac{13 \cdot 101}{77 \cdot 101} $$ = $$ \frac{13}{77} $$.