Bài 2 trang 11 sgk Toán 5, Bài 2. Tính:...

Bài 2. Tính:

a) $\frac{9}{10}.\frac{5}{6}$ ;   b) $\frac{6}{25}:\frac{21}{20}$ ;    c) $\frac{40}{7}.\frac{14}{5}$ ;    d) $\frac{17}{13}:\frac{51}{26}$ .

a) $\frac{9}{10}.\frac{5}{6}$ $= \frac{9.5}{10.6}=\frac{3.3.5}{2.5.2.3}=\frac{3}{4}$ ;

b) $\frac{6}{25}:\frac{21}{20}$ $= \frac{6}{25}.\frac{20}{21}=\frac{3.2.4.5}{5.5.7.3}=\frac{8}{35}$ ;

c) $\frac{40}{7}.\frac{14}{5}$ $=\frac{40.14}{7.5}=\frac{4.5.2.2.7}{5.7}=16$ ;

d) $\frac{17}{13}:\frac{51}{26}$ $\frac{17}{13}.\frac{26}{51}=\frac{17.2.13}{13.3.17}=\frac{2}{3}$ .