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