\[\boxed{\mathbf{2.2}\mathbf{98}}\]
\[\textbf{а)}\ \frac{3}{4} = \frac{5}{4} - \frac{2}{4}\]
\[\frac{3}{4} = \frac{12}{16} = \frac{15}{16} - \frac{3}{16}\]
\[\frac{3}{4} = \frac{15}{20} = \frac{18}{20} - \frac{3}{20}\]
\[\textbf{б)}\ \frac{3}{4} = \frac{1}{4} + \frac{2}{4}\]
\[\frac{3}{4} = \frac{12}{16} = \frac{9}{16} + \frac{3}{16}\]
\[\frac{3}{4} = \frac{15}{20} = \frac{9}{20} + \frac{6}{20}\]