Tính giá trị các biểu thức sau :
a) \(A = {5 \over {11}}.{{ - 3} \over {23}}.{{11} \over 5}\)
b) \(B = {{ - 7} \over 9}.{{13} \over {25}} - {{13} \over {25}}.{2 \over 9}\)
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c) \(C = {5 \over {11}}.{3 \over 7} + {5 \over {11}}.{4 \over 7} - {3 \over {11}}\)
\(\eqalign{ & a)A = {5 \over {11}}.{{ - 3} \over {23}}.{{11} \over 5} = \left( {{5 \over {11}}.{{11} \over 5}} \right).{{ - 3} \over {23}} = 1.{{ - 3} \over {23}} = {{ - 3} \over {23}}. \cr & b)B = {{ - 7} \over 9}.{{13} \over {25}} - {{13} \over {25}}.{2 \over 9} = {{13} \over {25}}.\left( {{{ - 7} \over 9} - {2 \over 9}} \right) = {{13} \over {25}}.( - 1) = {{ - 13} \over {25}}. \cr & c)C = {5 \over {11}}.{3 \over 7} + {5 \over {11}}.{4 \over 7} - {3 \over {11}} = {5 \over {11}}.\left( {{3 \over 7} + {4 \over 7}} \right) - {3 \over {11}} = {5 \over {11}}.1 - {3 \over {11}} = {5 \over {11}} - {3 \over {11}} = {2 \over {11}}. \cr} \)