Tính bằng hai cách: (6/11 + 5/11) x 3/7

Giải Toán lớp 4 Bài: Ôn tập về các phép tính với phân số (tiếp theo)

Toán lớp 4 trang 169 Bài 1:

a) $\left(\frac{6}{11}+\frac{5}{11}\right)\times \frac{3}{7}$                                      

b) $\frac{3}{5}\times \frac{7}{9}-\frac{3}{5}\times \frac{2}{9}$ 

c) $\left(\frac{6}{7}-\frac{4}{7}\right):\frac{2}{5}$                                          

d) $\frac{8}{15}:\frac{2}{11}+\frac{7}{15}:\frac{2}{11}$ 

Lời giải

a)    

Cách 1: $\left(\frac{6}{11}+\frac{5}{11}\right)\times \frac{3}{7}=\frac{11}{11}\times \frac{3}{7}=\frac{3}{7}$ 

Cách 2: $\left(\frac{6}{11}+\frac{5}{11}\right)\times \frac{3}{7}=\frac{6}{11}\times \frac{3}{7}+\frac{5}{11}\times \frac{3}{7}$$=\frac{18}{77}+\frac{15}{77}=\frac{33}{77}=\frac{3}{7}$ 

b) 

Cách 1: $\frac{3}{5}\times \frac{7}{9}-\frac{3}{5}\times \frac{2}{9}$$=\frac{21}{45}-\frac{6}{45}=\frac{15}{45}=\frac{1}{3}$

Cách 2: $\frac{3}{5}\times \frac{7}{9}-\frac{3}{5}\times \frac{2}{9}$$=\frac{3}{5}\times \left(\frac{7}{9}-\frac{2}{9}\right)=\frac{3}{5}\times \frac{5}{9}=\frac{1}{3}$ 

c)    

Cách 1: $\left(\frac{6}{7}-\frac{4}{7}\right):\frac{2}{5}=\frac{2}{7}:\frac{2}{5}=\frac{2}{7}\times \frac{5}{2}=\frac{5}{7}$ 

Cách 2: $\left(\frac{6}{7}-\frac{4}{7}\right):\frac{2}{5}=\frac{6}{7}:\frac{2}{5}-\frac{4}{7}:\frac{2}{5}$$=\frac{6}{7}\times \frac{5}{2}-\frac{4}{7}\times \frac{5}{2}=\frac{30}{14}-\frac{20}{14}$$=\frac{10}{14}=\frac{5}{7}$ 

d)   

Cách 1: $\frac{8}{15}:\frac{2}{11}+\frac{7}{15}:\frac{2}{11}$$=\frac{8}{15}\times \frac{11}{2}+\frac{7}{15}\times \frac{11}{2}$$=\frac{88}{30}+\frac{77}{30}=\frac{165}{30}=\frac{11}{2}$ 

Cách 2: $\frac{8}{15}:\frac{2}{11}+\frac{7}{15}:\frac{2}{11}$$=\left(\frac{8}{15}+\frac{7}{15}\right):\frac{2}{11}=\frac{15}{15}\times \frac{11}{2}=\frac{11}{2}$