Viết tiếp vào chỗ chấm: Nhận xét: 2/3 x 4/5 = ...

Giải Toán lớp 4 Bài: Luyện tập

Toán lớp 4 trang 134 Bài 1:

a) Viết tiếp vào chỗ chấm:

- Nhận xét: $\frac{2}{3}\times \frac{4}{5}=\cdots ;\frac{4}{3}\times \frac{2}{3}=\cdots$                                     

Vậy: $\frac{2}{3}\times \frac{4}{5}\dots \frac{4}{5}\times \frac{2}{3}$ 

- Nhận xét:  $\left(\frac{1}{3}\times \frac{2}{5}\right)\times \frac{3}{4}=\cdots ;\frac{1}{3}\times \left(\frac{2}{5}\times \frac{3}{4}\right)=\cdots$                            

Vậy: $\left(\frac{1}{3}\times \frac{2}{5}\right)\times \frac{3}{4}\dots \frac{1}{3}\times \left(\frac{2}{5}\times \frac{3}{4}\right)$ 

- Nhận xét:

 $\left(\frac{1}{5}\times \frac{2}{5}\right)\times \frac{3}{4}=\cdots ;\frac{1}{5}\times \frac{3}{4}+\frac{2}{5}\times \frac{3}{4}=\cdots$                              

Vậy: $\left(\frac{1}{5}+\frac{2}{5}\right)\times \frac{3}{4}\dots \frac{1}{5}\times \frac{3}{4}+\frac{2}{5}\times \frac{3}{4}$

b) Tính bằng hai cách:

$\frac{3}{22}\times \frac{3}{11}\times 22\left(\frac{1}{2}+\frac{1}{3}\right)\times \frac{2}{5}\frac{3}{5}\times \frac{17}{21}+\frac{17}{21}\times \frac{2}{5}$

Lời giải

a)

+) $\frac{2}{3}\times \frac{4}{5}=\frac{2\times 4}{3\times 5}=\frac{8}{15}\frac{4}{5}\times \frac{2}{3}=\frac{4\times 2}{5\times 3}=\frac{8}{15}$                                         

Vậy: $\frac{2}{3}\times \frac{4}{5}=\frac{4}{5}\times \frac{2}{3}$

+) $\left(\frac{1}{3}\times \frac{2}{5}\right)\times \frac{3}{4}=\frac{2}{15}\times \frac{3}{4}=\frac{6}{60}\frac{1}{3}\times \left(\frac{2}{5}\times \frac{3}{4}\right)=\frac{1}{3}\times \frac{6}{20}=\frac{6}{60}$                              

Vậy: $\left(\frac{1}{3}\times \frac{2}{5}\right)\times \frac{3}{4}=\frac{1}{3}\times \left(\frac{2}{5}\times \frac{3}{4}\right)$  

+)  $\left(\frac{1}{5}+\frac{2}{5}\right)\times \frac{3}{4}=\frac{3}{5}\times \frac{3}{4}=\frac{9}{20}\frac{1}{5}\times \frac{3}{4}+\frac{2}{5}\times \frac{3}{4}=\frac{3}{20}+\frac{6}{20}=\frac{9}{20}$                              

Vậy: $\left(\frac{1}{5}+\frac{2}{5}\right)\times \frac{3}{4}=\frac{1}{5}\times \frac{3}{4}+\frac{2}{5}\times \frac{3}{4}$

b)

+)  $\frac{3}{22}\times \frac{3}{11}\times 22=\left(\frac{3}{22}\times \frac{3}{11}\right)\times 22$$=\frac{9}{242}\times 22=\frac{198}{242}=\frac{9}{11}$

$\frac{3}{22}\times \frac{3}{11}\times 22=\frac{3}{22}\times \left(\frac{3}{11}\times 22\right)$$=\frac{3}{22}\times \frac{66}{11}=\frac{198}{242}=\frac{9}{11}$

+) $\left(\frac{1}{2}+\frac{1}{3}\right)\times \frac{2}{5}=\frac{5}{6}\times \frac{2}{5}=\frac{10}{30}=\frac{1}{3}$ 

$\left(\frac{1}{2}+\frac{1}{3}\right)\times \frac{2}{5}=\frac{1}{2}\times \frac{2}{5}+\frac{2}{5}\times \frac{1}{3}$$=\frac{2}{10}+\frac{2}{15}=\frac{10}{30}=\frac{1}{3}$

+)  $\frac{3}{5}\times \frac{17}{21}+\frac{17}{21}\times \frac{2}{5}$$=\frac{51}{105}+\frac{34}{105}=\frac{85}{105}=\frac{17}{21}$

$\frac{3}{5}\times \frac{17}{21}+\frac{17}{21}\times \frac{2}{5}=\frac{3}{5}\times \frac{17}{21}+\frac{2}{5}\times \frac{17}{21}$$=\left(\frac{3}{5}+\frac{2}{5}\right)\times \frac{17}{21}=\frac{5}{5}\times \frac{17}{21}$$=1\times \frac{17}{21}=\frac{17}{21}$