\[Стр.\ 18.\]
\[\boxed{\mathbf{19.}}\]
\[\textbf{а)}\ \frac{5}{a}\ \ и\ \frac{a}{11}\]
\[a = 6;7;8;9;10.\]
\[\textbf{б)}\ \frac{a}{13}\ и\ \frac{14}{a}\]
\[a = 14.\]
\[\textbf{в)}\ \frac{8}{a}\ и\ \frac{a}{12}\]
\[a = 9;10;11.\]
\[\textbf{г)}\ \frac{12}{a}\ и\ \frac{a}{8}\]
\[a = 8;\ 9;10;11;12.\]
\[\boxed{\mathbf{20.}}\]
\[\textbf{а)}\ \frac{1}{2} < \frac{a}{10} < 1\]
\[\frac{5}{10} < \frac{a}{10} < \frac{10}{10}\]
\[a = 6;7;8;9.\]
\[\textbf{б)}\ \frac{1}{3} < \frac{a}{6} < 1\]
\[\frac{2}{6} < \frac{a}{6} < \frac{6}{6}\]
\[a = 3;4;5.\]
\[\textbf{в)}\ \frac{1}{6} < \frac{a}{24} < \frac{1}{3}\]
\[\frac{4}{24} < \frac{a}{24} < \frac{8}{24}\]
\[a = 5;6;7.\]
\[\textbf{г)}\ \frac{1}{4} < \frac{a}{16} < \frac{1}{2}\]
\[\frac{4}{16} < \frac{a}{16} < \frac{8}{16}\]
\[a = 5;6;7.\]
\[29.\ Сложение\ и\ вычитание\ дробей\ с\ одинаковыми\ знаменателями\]
\[\boxed{\mathbf{1.}}\]
\[\textbf{а)}\ числители;знаменатель;\]
\[\textbf{б)}\ числителя,\ числитель,\ \]
\[знаменатель;\]