a) \({3 \over 7} + ... = {{ - 2} \over 7}\) ;
b) \(... + {{ - 4} \over {11}} = {{ - 7} \over {11}}\) ;
c) \({{ - 6} \over {18}} + ... = {{ - 3} \over {18}}\) ;
d) \({{ - 5} \over {13}} + ... = {{ - 5} \over {13}}\).
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\(\eqalign{ & a){3 \over 7} + ... = {2 \over 7} \cr & ... = {{ - 2} \over 7} - {3 \over 7} = {{ - 5} \over 7} \cr} \)
Vậy \({3 \over 7} + {{ - 5} \over 7} = {{ - 2} \over 7}\)
\(\eqalign{ & b)... + {{ - 4} \over {11}} = {{ - 7} \over {11}} \cr & ... = {{ - 7} \over {11}} + {4 \over {11}} = {{ - 3} \over {11}} \cr & \Rightarrow {{ - 3} \over {11}} + {{ - 4} \over {11}} = {{ - 7} \over {11}} \cr & c){{ - 6} \over {18}} + ... = {{ - 3} \over {18}} \cr & ... = {{ - 3} \over {18}} + {6 \over {18}} = {3 \over {18}} = {1 \over 6} \cr & \Rightarrow {{ - 6} \over {18}} + {1 \over 6} = {{ - 3} \over {18}}. \cr & d){{ - 5} \over {13}} + ... = {{ - 5} \over {13}} \cr & ... = {{ - 5} \over {13}} + {5 \over {13}} = 0 \cr & \Rightarrow {{ - 5} \over {13}} + 0 = {{ - 5} \over {13}}. \cr} \)