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\[\boxed{\mathbf{3.}}\]
\[\textbf{а)}\frac{c}{b} = \frac{b}{c}\]
\[\textbf{б)}\frac{2x}{5y} = \frac{5y}{2x}\]
\[\textbf{в)}\frac{1}{x} = x\]
\[\textbf{г)}\frac{a}{7} = \frac{7}{a}\]
\[\boxed{\mathbf{4.}}\]
\[умножить;обратное.\]
\[\boxed{\mathbf{5.}}\]
\[3 \rightarrow \frac{1}{3}\]
\[5 \rightarrow \frac{1}{5}\]
\[0,2 = \frac{1}{5} \rightarrow 5\]
\[0,5 = \frac{1}{2} \rightarrow 2\]
\[\frac{1}{7} \rightarrow 7\]
\[\frac{1}{130} \rightarrow 130\]
\[2\frac{3}{4} = \frac{11}{4} \rightarrow \frac{4}{11}\]
\[\frac{87}{6} \rightarrow \frac{6}{87}\]
\[\frac{200}{203} \rightarrow \frac{203}{200}\]
\[5,3 = \frac{53}{10} \rightarrow \frac{10}{53}\]
\[4\frac{5}{8} = \frac{37}{8} \rightarrow \frac{8}{37}\]
\[\frac{3}{5} \rightarrow \frac{5}{3}\]
\[\boxed{\mathbf{6.}}\]
\[\textbf{а)}\ \frac{9}{16}\ :6 = \frac{9}{16} \cdot \frac{1}{6}\ \]
\[12\ :\frac{5}{8} = 12 \cdot \frac{8}{5}\]
\[\textbf{б)}\ \frac{3}{10}\ :\frac{8}{11} = \frac{3}{10} \cdot \frac{11}{8}\]
\[\frac{7}{12}\ :3\frac{1}{16} = \frac{7}{12}\ :\frac{49}{16} = \frac{7}{12} \cdot \frac{16}{49}\]
\[\boxed{\mathbf{7.}}\]
\[\textbf{а)}\ 1\ :\frac{1}{7} = 1 \cdot 7 = 7\]
\[5\ :\frac{5}{8} = 5 \cdot \frac{8}{5} = 8\]
\[4\ :\frac{8}{9} = 4 \cdot \frac{9}{8} = \frac{9}{2} = 4,5\]
\[120\ :\frac{1}{10} = 120 \cdot 10 = 1200\]
\[\textbf{б)}\ \frac{3}{7}\ :\frac{9}{14} = \frac{3}{7} \cdot \frac{14}{9} = \frac{2}{3}\]
\[\frac{3}{10}\ :\frac{8}{11} = \frac{3}{10} \cdot \frac{11}{8} = \frac{33}{80}\]
\[\frac{7}{12}\ :\frac{5}{6} = \frac{7}{12} \cdot \frac{6}{5} = \frac{7}{10}\]
\[\frac{7}{8}\ :\frac{3}{4} = \frac{7}{8} \cdot \frac{4}{3} = \frac{7}{6} = 1\frac{1}{6}\]
\[\textbf{в)}\ 12\ :\frac{5}{8} = 12 \cdot \frac{8}{5} = 13\frac{3}{5}\]
\[\frac{1}{3}\ :3 = \frac{1}{3} \cdot \frac{1}{3} = \frac{1}{9}\]
\[\frac{5}{7}\ :10 = \frac{5}{7} \cdot \frac{1}{10} = \frac{5}{70} = \frac{1}{14}\]
\[\frac{10}{11}\ :5 = \frac{10}{11} \cdot \frac{1}{5} = \frac{2}{11}\]
\[\textbf{г)}\frac{1}{200}\ :200 = \frac{1}{200} \cdot \frac{1}{200} =\]
\[= \frac{1}{40000}\]
\[\frac{9}{16}\ :6 = \frac{9}{16} \cdot \frac{1}{6} = \frac{3}{32}\]
\[\frac{84}{121}\ :\frac{63}{110} = \frac{84}{121} \cdot \frac{110}{63} =\]
\[= \frac{28 \cdot 10}{11 \cdot 21} = \frac{40}{33} = 1\frac{7}{33}\]