Tính nhanh :
a) \(B = {{ - 2} \over {13}} + {{14} \over {23}} + {{ - 11} \over {13}} + {5 \over {17}} + {9 \over {23}}.\)
b) \(C = {{ - 3} \over 6} + {3 \over {15}} + {{ - 1} \over 3} + {{ - 5} \over {30}}\).
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\(\eqalign{ & a)B = {{ - 2} \over {13}} + {{14} \over {23}} + {{ - 11} \over {13}} + {5 \over {17}} + {9 \over {23}} = \left( {{{ - 2} \over {13}} + {{ - 11} \over {13}}} \right) + \left( {{{14} \over {23}} + {9 \over {23}}} \right) + {5 \over {17}} = - 1 + 1 + {5 \over {17}} = {5 \over {17}} \cr & b)C = {{ - 3} \over 6} + {3 \over {15}} + {{ - 1} \over 3} + {{ - 5} \over {30}} = {{ - 1} \over 2} + {1 \over 5} + {{ - 1} \over 3} + {{ - 1} \over 6} = {{ - 3} \over 6} + {1 \over 5} + {{ - 2} \over 6} + {{ - 1} \over 6} \cr & = \left( {{{ - 3} \over 6} + {{ - 2} \over 6} + {{ - 1} \over 6}} \right) + {3 \over {15}} = - 1 + {1 \over 5} = {{ - 5} \over 5} + {1 \over 5} = {{ - 4} \over 5} \cr} \)