Đề bài

a) \({{ - 5} \over 9} + {2 \over {11}} + {{ - 4} \over {11}}\) ;

b) \({{ - 5} \over {24}} + {{ - 7} \over {24}} + {9 \over {18}}\) ;

c ) \({{ - 4} \over {13}} + \left( {{{ - 9} \over {13}} + 1} \right)\) ;

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

Lời giải chi tiết

\(\eqalign{  & a){{ - 5} \over 9} + {2 \over {11}} + {{ - 4} \over {11}} = {{ - 5} \over 9} + {{2 - 4} \over {11}} = {{ - 5} \over 9} + {{ - 2} \over {11}} = {{ - 55} \over {99}} + {{ - 18} \over {99}} = {{ - 73} \over {99}}.  \cr  & b){{ - 5} \over {24}} + {{ - 7} \over {24}} + {9 \over {18}} = {{( - 5) + ( - 7)} \over {24}} + {1 \over 2} = {{ - 12} \over {24}} + {1 \over 2} = {{ - 1} \over 2} + {1 \over 2} = 0.  \cr  & c){{ - 4} \over {13}} + \left( {{{ - 9} \over {13}} + 1} \right) = {{ - 4} \over {13}} - {9 \over {13}} + 1 = {{ - 4 - 9} \over {13}} + 1 = {{ - 13} \over {13}} + 1 =  - 1 + 1 = 0.  \cr  & d){5 \over 9} + {{ - 2} \over 7} + {4 \over 9} + {{ - 5} \over 7} + {2 \over 3} = \left( {{5 \over 9} + {4 \over 9}} \right) + \left( {{{ - 2} \over 7} + {{ - 5} \over 7}} \right) + {2 \over 3}  \cr  &  = {{5 + 4} \over 9} + {{ - 2 - 5} \over 7} + {2 \over 3} = {9 \over 9} + {{ - 7} \over 7} + {2 \over 3} = 1 + ( - 1) + {2 \over 3} = 0 + {2 \over 3} = {2 \over 3}. \cr} \)

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