Câu 1: Trang 134 sách giáo khoa (SGK) toán tiếng Anh lớp 4a) Fill in the blanks with the correct...

a) Cách làm chi tiết cho câu hỏi trên:

1.
Nhận xét:
$\frac{2}{3} x \frac{4}{5} = \frac{2\times 4}{3 \times 5} = \frac{8}{15}$
$\frac{4}{5} x \frac{2}{3} = \frac{4\times 2}{5 \times 3} = \frac{8}{15}$
Vậy: $\frac{2}{3} x \frac{4}{5} = \frac{4}{5} x \frac{2}{3} = \frac{8}{15}$

2.
Nhận xét:
$(\frac{1}{3} x \frac{2}{5}) x \frac{3}{4} = \frac{1 \times 2}{3 \times 5} x \frac{3}{4} = \frac{2}{15} x \frac{3}{4} = \frac{2\times 3}{15 \times 4} = \frac{6}{60}$
$\frac{1}{3} x (\frac{2}{5} x \frac{3}{4}) = \frac{1}{3} x \frac{2 \times 3}{5 \times 4} = \frac{1}{3} x \frac{6}{20} = \frac{1 \times 6}{3 \times 20} = \frac{6}{60}$
Vậy: $(\frac{1}{3} x \frac{2}{5}) x \frac{3}{4} = \frac{1}{3} x (\frac{2}{5} x \frac{3}{4}) = \frac{6}{60}$

3.
Nhận xét:
$(\frac{1}{5} + \frac{2}{5}) x \frac{3}{4} = \frac{1 + 2}{5} x \frac{3}{4} = \frac{3}{5} x \frac{3}{4} = \frac{3\times 3}{5 \times 4} = \frac{9}{20}$
$\frac{1}{5} x \frac{3}{4} + \frac{2}{5} x \frac{3}{4} = \frac{1\times 3}{5 \times 4} + \frac{2\times 3}{5 \times 4} = \frac{3}{20} + \frac{6}{20} = \frac{9}{20}$
Vậy: $(\frac{1}{5} + \frac{2}{5}) x \frac{3}{4} = \frac{1}{5} x \frac{3}{4} + \frac{2}{5} x \frac{3}{4} = \frac{9}{20}$

b) Tính theo hai cách:
1. $\frac{3}{22} x \frac{3}{11} x 22$
Cách 1: $\frac{3}{22} x \frac{3}{11} x 22 = \frac{3 \times 3 \times 22}{22 \times 11} = \frac{9}{11}$
Cách 2: $\frac{3}{22} x \frac{3}{11} x 22 = \frac{3}{22} x (\frac{3}{11} x 22) = \frac{3}{22} x \frac{3 \times 22}{11} = \frac{3}{22} x \frac{3 \times 22}{11} = \frac{3}{22} x \frac{3 \times 11 \times 2}{11} = \frac{3}{22} x \frac{6}{1} = \frac{9}{11}$

2. $(\frac{1}{2} + \frac{1}{3}) x \frac{2}{5}$
Cách 1: $(\frac{1}{2} + \frac{1}{3}) x \frac{2}{5} = \frac{5}{6} x \frac{2}{5} = \frac{5 \times 2}{6 \times 5} = \frac{2}{6} = \frac{1}{3}$
Cách 2: $(\frac{1}{2} + \frac{1}{3}) x \frac{2}{5} = \frac{1}{2} x \frac{2}{5} + \frac{1}{3} x \frac{2}{5} = \frac{1 \times 2}{2 \times 5} + \frac{2 \times 1}{5 \times 3} = \frac{2}{10} + \frac{2}{15} = \frac{6}{30} + \frac{4}{30} = \frac{10}{30} = \frac{1}{3}$

3. $\frac{3}{5} x \frac{17}{21} + \frac{17}{21} x \frac{2}{5}$
Cách 1: $\frac{3}{5} x \frac{17}{21} + \frac{17}{21} x \frac{2}{5} = \frac{3 \times 17}{5 \times 21} + \frac{17 \times 2}{21 \times 5} = \frac{51}{105} + \frac{34}{105} = \frac{85}{105} = \frac{17}{21}$
Cách 2: $\frac{3}{5} x \frac{17}{21} + \frac{17}{21} x \frac{2}{5} = (\frac{3}{5} + \frac{2}{5} ) x \frac{17}{21} = \frac{5}{5} x \frac{17}{21} = 1 x \frac{17}{21} = \frac{17}{21}$