Tính: 2/7 + 3/5

Giải Toán lớp 4 Bài: Ôn tập về các phép tính với phân số

Toán lớp 4 trang 167 Bài 2:

a)  

$\frac{2}{7}+\frac{3}{5};\frac{31}{35}-\frac{2}{7}\frac{31}{35}-\frac{3}{5};\frac{3}{5}+\frac{2}{7}$                                                                     

b)

$\frac{3}{4}+\frac{1}{6};\frac{11}{12}-\frac{3}{4}\frac{11}{12}-\frac{1}{6};\frac{1}{6}+\frac{3}{4}$                                                                       

Lời giải

a)                                      

 $\frac{2}{7}+\frac{3}{5}=\frac{10}{35}+\frac{21}{35}=\frac{31}{35}\frac{31}{35}-\frac{2}{7}=\frac{31}{35}-\frac{10}{35}=\frac{21}{35}=\frac{3}{5}\frac{31}{35}-\frac{3}{5}=\frac{31}{35}-\frac{21}{35}=\frac{10}{35}=\frac{2}{7}\frac{3}{5}+\frac{2}{7}=\frac{21}{35}+\frac{10}{35}=\frac{31}{35}$                        

b)                                      

$\frac{3}{4}+\frac{1}{6}=\frac{9}{12}+\frac{2}{12}=\frac{11}{12}\frac{11}{12}-\frac{3}{4}=\frac{11}{12}-\frac{9}{12}=\frac{2}{12}=\frac{1}{6}\frac{11}{12}-\frac{1}{6}=\frac{11}{12}-\frac{2}{12}=\frac{9}{12}=\frac{3}{4}\frac{1}{6}+\frac{3}{4}=\frac{2}{12}+\frac{9}{12}=\frac{11}{12}$