\[\begin{array}{l}
A)7/9{\rm{ ;}}2/5{\rm{ }}\\
\frac{7}{9} = \frac{{7 \times 5}}{{9 \times 5}} = \frac{{35}}{{45}}\\
\frac{2}{5} = \frac{{2 \times 9}}{{5 \times 9}} = \frac{{18}}{{45}}\\
B)6/7{\rm{ ;}}17/21{\rm{ }}\\
\frac{6}{7} = \frac{{6 \times 3}}{{7 \times 3}} = \frac{{18}}{{21}}\\
\frac{{17}}{{21}}\\
C)3/8{\rm{ ;}}5/6\\
\frac{3}{8}{\rm{ = }}\frac{{3 \times 3}}{{8 \times 3}}{\rm{ = }}\frac{9}{{24}}\\
\frac{5}{6}{\rm{ = }}\frac{{5 \times 4}}{{6 \times 4}}{\rm{ = }}\frac{{20}}{{24}}{\rm{ }}\\
D)5/6;3/4{\rm{ ;}}7/8\\
\frac{5}{6} = \frac{{5 \times 4}}{{6 \times 4}} = \frac{{20}}{{24}}\\
\frac{3}{4} = \frac{{3 \times 6}}{{4 \times 6}} = \frac{{18}}{{24}}\\
\frac{7}{8} = \frac{{7 \times 3}}{{8 \times 3}} = \frac{{21}}{{24}}
\end{array}\]

A)7/9 và2/5     $\frac{35}{45}và\frac{18}{45}$

B)6/7 và17/21     $\frac{18}{21}và\frac{17}{21}$

C)3/8 và5/6    $\frac{9}{24} và \frac{20}{24}$

D)5/6;3/4 và7/8   $\frac{20}{24};\frac{18}{24};\frac{21}{24}$