Tính phép tính: a) 7/8 . ( 2/12 + 4/10)

 a) \(\frac{7}{8}\,\,.\,\,\left( {\frac{2}{{12}}\, + \,\frac{4}{{10}}} \right) = \frac{7}{8}\left( {\frac{{10}}{{60}} + \frac{{24}}{{60}}} \right)\)

 \[ = \frac{7}{8}.\frac{{17}}{{30}}\]=\(\frac{{119}}{{240}}\)

 b) \(1\frac{3}{7}:\left( { - \frac{4}{5}} \right) - 3\frac{3}{7}:\left( { - \frac{4}{5}} \right) = \left( {\frac{{10}}{7} - \frac{{24}}{7}} \right):\left( { - \frac{4}{5}} \right)\)                            

 \( = \left( {\frac{{10}}{7} - \frac{{24}}{7}} \right).\frac{{ - 5}}{4}\)\[ = \frac{{ - 5}}{4}.( - 2) = \frac{5}{2}\]  

c) \(\frac{{{9^2}\,.\,{{27}^3}}}{{{{81}^4}}} = \frac{{{{({3^2})}^2}\,.\,{{({3^3})}^3}}}{{{{({3^4})}^4}}} = \frac{{{3^4}\,.\,{3^9}}}{{{3^{16}}}}\)

           \[ = \frac{{{3^{13}}}}{{{3^{16}}}} = \frac{1}{{27}}\]

 d) \[2,5\,\,.\,\,{\left( { - \frac{1}{2}} \right)^3}.\,\,0,4{\rm{ + 41}}\frac{2}{7}.\frac{3}{8} - 38\frac{2}{7}.\frac{3}{8}\]

\[ = (2,5\,.\,0,4)\,\,.\,\,\left( { - \frac{1}{8}} \right) + \frac{3}{8}\,\,\left( {41\frac{2}{7} - 38\frac{2}{7}} \right)\]

 \( = - \frac{1}{8} + \frac{3}{8}\,.\,3 = - \frac{1}{8} + \frac{9}{8} = 1\)