Giải sbt toán 6 tập 2: bài tập 86 trang 25

a)    \({2 \over 3} + {1 \over 5}.{{10} \over 7} \)

\(= {2 \over 3} + {{1.10} \over {5.7}} \)

\(= {2 \over 3} + {2 \over 7} \)

\(= {{14} \over {21}} + {6 \over {21}}\)

\(= {{20} \over {21}}\) 

b)    \({7 \over {12}} - {{27} \over 7}.{1 \over {18}} \)

\(= {7 \over 12} - {{27.1} \over {7.18}} \)

\(= {7 \over 12} - {3 \over {14}} \)

\(= {{49} \over {84}} + {{ - 18} \over {84}} \)

\(= {{31} \over {14}} \)

c)     \(\left( {{{23} \over {41}} - {{15} \over {82}}} \right).{{41} \over {25}} \)

\(= \left( {{{46} \over {82}} + {{ - 15} \over {82}}} \right).{{41} \over {25}} \)

\(= {{31} \over {82}}.{{41} \over {25}} \)

\(= {{31.41} \over {82.25}} = {{31} \over {50}}\) 

d)    \(\left( {{4 \over 5} + {1 \over 2}} \right).\left( {{3 \over {13}} - {8 \over {13}}} \right) \)

\(= \left( {{8 \over {10}} + {5 \over {10}}} \right).\left( {{3 \over {13}} + {{ - 8} \over {13}}} \right)\)

\(= {{13} \over {10}}.{{ - 5} \over {13}} \)

\(= {{13.( - 5)} \over {10.13}} \)

\(= {{ - 1} \over 2}\)