Tính bằng cách thuận tiện nhất
a) \({5 \over {14}} \times {7 \over {13}} \times {{26} \over {25}}\); b) \({5 \over {14}} \times {7 \over {13}} \times {{26} \over {25}}\).
a) \({{21} \over {11}} \times {{22} \over {17}} \times {{68} \over {63}} = {{21 \times 22 \times 68} \over {11 \times 17 \times 63}}\)\(= {{21 \times 11 \times 2 \times 17 \times 4} \over {11 \times 17 \times 21 \times 3}}= {{2 \times 4} \over 3}={8 \over 3}\);
b) \({5 \over {14}} \times {7 \over {13}} \times {{26} \over {25}} ={{5 \times 7 \times 26} \over {14 \times 13 \times 25}}\)\(= {{5 \times 7 \times 13 \times 2 } \over {7 \times 2 \times 13 \times 5 \times 5 }} = {1 \over 5}\).
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