Tính: \({5 \over 9} + {{ - 2} \over 7} + {4 \over 9} + {{ - 5} \over 7} + {2 \over 3}\).

* Hướng dẫn giải

\({5 \over 9} + {{ - 2} \over 7} + {4 \over 9} + {{ - 5} \over 7} + {2 \over 3} \)

\(= \left( {{5 \over 9} + {4 \over 9}} \right) + \left( {{{ - 2} \over 7} + {{ - 5} \over 7}} \right) + {2 \over 3} \)

\(= {{5 + 4} \over 9} + {{ - 2 - 5} \over 7} + {2 \over 3} \)

\(= {9 \over 9} + {{ - 7} \over 7} + {2 \over 3} \)

\(= 1 + ( - 1) + {2 \over 3} \)

\(= 0 + {2 \over 3} = {2 \over 3}.\)