Giải câu 1 trang 43 sách toán VNEN lớp 4 tập 2

a.  $\frac{1}{2}+\frac{1}{3}$ ;  $\frac{2}{5}+\frac{3}{2}$   ;   $\frac{2}{5}+\frac{3}{4}$

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

b. $\frac{7}{2}+\frac{1}{4}$   ; $\frac{5}{6}+\frac{5}{3}$   ; $\frac{3}{4}+\frac{1}{8}$

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

c. $\frac{1}{6}+\frac{7}{9}$    ; $\frac{5}{8}+\frac{5}{6}$  ; $\frac{3}{4}+\frac{5}{6}$

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