B. Hoạt động ứng dụngCâu 1:Trang 43 sách toán VNEN lớp 4 tập 2Tính (theo mẫu):a....

Để tính các phép cộng phân số theo mẫu, ta thực hiện như sau:

a.
- $\frac{1}{2}+\frac{1}{3} = \frac{1\times 3}{2\times 3} + \frac{1\times 2}{3\times 2} = \frac{3}{6} + \frac{2}{6} = \frac{5}{6}$
- $\frac{2}{5}+\frac{3}{2} = \frac{2\times 2}{5\times 2} + \frac{3\times 5}{2\times 5} = \frac{4}{10} + \frac{15}{10} = \frac{19}{10}$
- $\frac{2}{5}+\frac{3}{4} = \frac{2\times 4}{5\times 4} + \frac{3\times 5}{4\times 5} = \frac{8}{20} + \frac{15}{20} = \frac{23}{20}$

b.
- $\frac{7}{2}+\frac{1}{4} = \frac{7\times 2}{2\times 2} + \frac{1}{4} = \frac{14}{4} + \frac{1}{4} = \frac{15}{4}$
- $\frac{5}{6}+\frac{5}{3} = \frac{5}{6} + \frac{5\times 2}{3\times 2} = \frac{5}{6} + \frac{10}{6} = \frac{15}{6}$
- $\frac{3}{4}+\frac{1}{8} = \frac{3\times 2}{4\times 2} + \frac{1}{8} = \frac{6}{8} + \frac{1}{8} = \frac{7}{8}$

c.
- $\frac{1}{6}+\frac{7}{9} = \frac{1\times 3}{6\times 3} + \frac{7\times 2}{9\times 2} = \frac{3}{18} + \frac{14}{18} = \frac{17}{18}$
- $\frac{5}{8}+\frac{5}{6} = \frac{5\times 3}{8\times 3} + \frac{5\times 4}{6\times 4} = \frac{15}{24} + \frac{20}{24} = \frac{35}{24}$
- $\frac{3}{4}+\frac{5}{6} = \frac{3\times 3}{4\times 3} + \frac{5\times 2}{6\times 2} = \frac{9}{12} + \frac{10}{12} = \frac{19}{12}$

Vậy đáp án cho từng phép tính là:
- a. $\frac{1}{2}+\frac{1}{3} = \frac{5}{6}$, $\frac{2}{5}+\frac{3}{2} = \frac{19}{10}$, $\frac{2}{5}+\frac{3}{4} = \frac{23}{20}$
- b. $\frac{7}{2}+\frac{1}{4} = \frac{15}{4}$, $\frac{5}{6}+\frac{5}{3} = \frac{15}{6}$, $\frac{3}{4}+\frac{1}{8} = \frac{7}{8}$
- c. $\frac{1}{6}+\frac{7}{9} = \frac{17}{18}$, $\frac{5}{8}+\frac{5}{6} = \frac{35}{24}$, $\frac{3}{4}+\frac{5}{6} = \frac{19}{12}$