Tính. ( frac{{39}}{{45}} ) + ( frac{{13}}{{15}} ) = ……………………………… ( frac{{21}}{{18}} ) – ( frac{{10}}{9} ) = ……………………………… 9 × ( frac{5}{{63}} ) = ……………………………… ( frac{1}{7} ) : ( frac{5}{{28}} ) = …………...

( frac{{39}}{{45}} ) + ( frac{{13}}{{15}} ) = ( frac{{13}}{{15}} ) + ( frac{{13}}{{15}} ) = ( frac{{26}}{{15}} ) ( frac{{21}}{{18}} ) – ( frac{{10}}{9} ) = ( frac{{21}}{{18}} ) – ( frac{{20}}{{18}} ) = ( frac{1}{{18}} ) 9 × ( frac{5}{{63}} ) = ( frac{5}{7} ) ( frac{1}{7} ) : ( frac{5}{{28}} ) = ( frac{1}{7} ) × ( frac{{28}}{5} ) = ( frac{4}{5} )