Bài tập 2 trang 45 VBT Toán 4 tập 2

a)

Cách 1: \(\displaystyle {3 \over 4} \times {1 \over 2} \times 2 = {{3} \over {8}} \times 2 = {3 \over 4}\)

Cách 2: \(\displaystyle {3 \over 4} \times {1 \over 2} \times 2 = \dfrac{3}{4} \times \left( \dfrac{1}{2} \times 2 \right)\displaystyle {3 \over 4} \times 1 = {3 \over 4}\)

b)

Cách 1:

\(\displaystyle \left( {{3 \over 4} + {1 \over 2}} \right) \times {5 \over 7} \\ \displaystyle= \left( {{{3} \over 4}} + \dfrac{2}{4}\right) \times {5 \over 7} \\ \displaystyle = {{5} \over 4} \times {5 \over 7} \\\displaystyle = {{5\times 5} \over {4 \times 7}}= {{25} \over {28}}\)

Cách 2:

\(\displaystyle \left( {{3 \over 4} + {1 \over 2}} \right) \times {5 \over 7} \\= \displaystyle{3 \over 4} \times {5 \over 7} + {1 \over 2} \times {5 \over 7} \\\displaystyle = {{15} \over {28}} + {5 \over {14}} \\\displaystyle = {{15} \over {28}} + {{10} \over {28}} = {{25} \over {28}}\)

c)

Cách 1:

\(\displaystyle {5 \over 7} \times {{13} \over {21}} + {2 \over 7} \times {{13} \over {21}} \\\displaystyle = {{5 \times 13} \over {7 \times 21}} + {{2 \times 13} \over {7 \times 21}} \\\displaystyle = {{65} \over {147}} + {{26} \over {147}} \\\displaystyle = {{91} \over {147}} = {{13} \over {21}}\)

Cách 2: 

\(\displaystyle {5 \over 7} \times {{13} \over {21}} + {2 \over 7} \times {{13} \over {21}} \\\displaystyle = \left( {{5 \over 7} + {2 \over 7}} \right) \times {{13} \over {21}} \\=\displaystyle 1 \times {{13} \over {21}} = {{13} \over {21}}\)

-- Mod Toán lớp 4 HỌC247