Tính theo hai cách :
a) \(\displaystyle \left( {{6 \over {11}} + {5 \over {11}}} \right) \times {3 \over 7} \) b) \(\displaystyle {3 \over 5} \times {7 \over 9} - {3 \over 5} \times {2 \over 9} \)
a) \(\displaystyle \left( {{6 \over {11}} + {5 \over {11}}} \right) \times {3 \over 7}\)
Cách 1: \(\displaystyle \left( {{6 \over {11}} + {5 \over {11}}} \right) \times {3 \over 7} = {{11} \over {11}} \times {3 \over 7}\)\(\displaystyle =1 \times {3 \over 7} = {3 \over 7} \)
Cách 2: \(\displaystyle \left( {{6 \over {11}} + {5 \over {11}}} \right) \times {3 \over 7}\)\(\displaystyle = {6 \over {11}} \times {3 \over 7} + {5 \over {11}} \times {3 \over 7}={{18} \over {77}} + {{15} \over {77}} \)\(\displaystyle= {{33} \over {77}} = {3 \over 7}\)
b) \(\displaystyle {3 \over 5} \times {7 \over 9} - {3 \over 5} \times {2 \over 9}\)
Cách 1: \(\displaystyle {3 \over 5} \times {7 \over 9} - {3 \over 5} \times {2 \over 9}\)\(\displaystyle = {{21} \over {45}} - {6 \over {45}} = {{15} \over {45}} = {1 \over 3}\)
Cách 2: \(\displaystyle {3 \over 5} \times {7 \over 9} - {3 \over 5} \times {2 \over 9}\)\(\displaystyle = {3 \over 5} \times \left( {{7 \over 9} - {2 \over 9}} \right) = {3 \over 5} \times{5 \over 9}\)\(\displaystyle = {{15} \over {45}} = {1 \over 3}\)
-- Mod Toán lớp 4