Tính :
a) \(\displaystyle {2 \over 3} + {5 \over 9} - {3 \over 4}=............\)
b) \(\displaystyle {2 \over 7}:{2 \over 3} - {1 \over 7}=.............\)
c) \(\displaystyle {2 \over 5} \times {1 \over 4}:{3 \over 8}=.............\)
a) \(\displaystyle {2 \over 3} + {5 \over 9} - {3 \over 4} = {{6} \over 9}+ {5 \over 9}- {3 \over 4}\)\(\displaystyle ={{11} \over 9} - {3 \over 4} ={{44} \over 36} - {27 \over 36} = {{17} \over {36}}\)
b) \(\displaystyle {2 \over 7}:{2 \over 3} - {1 \over 7} = {2 \over 7} \times {3 \over 2} - {1 \over 7}\)\(\displaystyle ={3 \over 7} - {1 \over 7}={2 \over 7}\)
c) \(\displaystyle {2 \over 5} \times {1 \over 4}:{3 \over 8} = {2 \over 5} \times {1 \over 4} \times {8 \over 3} \) \(\displaystyle =\dfrac{2\times 1 \times 8}{5 \times 4\times 3}=\dfrac{2\times 1 \times 2\times 4}{5 \times 4\times 3}= {4 \over {15}}\)
-- Mod Toán lớp 4