Tính bằng cách thuận tiện nhất:
a)\( \displaystyle{{18} \over {15}} + {7 \over {15}} + {{12} \over {15}}\) b) \(\displaystyle {9 \over 7} + {8 \over 7} + {{11} \over 7}\)
a) \(\displaystyle {{18} \over {15}} + {7 \over {15}} + {{12} \over {15}} \)
\( \displaystyle = \left( {{{18} \over {15}} + {{12} \over {15}}} \right) + {7 \over {15}} \)
\(\displaystyle = {{30} \over {15}} + {7 \over {15}} = {{37} \over {15}} \)
b) \(\displaystyle {9 \over 7} + {8 \over 7} + {{11} \over 7} \)
\( \displaystyle = \left( {{9 \over 7} + {{11} \over 7}} \right) + {8 \over 7} \)
\(\displaystyle= {{20} \over 7} + {8 \over 7} = {{28} \over 7}=4 \)
-- Mod Toán lớp 4