Bài 2 trang 11 SGK Toán 5

Đề bài

Tính:

a) \( \frac{9}{10}\times\frac{5}{6}\) ;            b) \( \frac{6}{25}:\frac{21}{20}\) ;    

c) \( \frac{40}{7}\times\frac{14}{5}\) ;           d) \( \frac{17}{13}:\frac{51}{26}\) .

Hướng dẫn giải

a) \( \frac{9}{10} \times \frac{5}{6} = \frac{9 \times 5}{10\times 6}=\frac{3\times 3\times 5}{2\times 5 \times 2 \times 3}=\frac{3}{4}\) ;

b) \( \frac{6}{25}:\frac{21}{20} = \frac{6}{25}\times \frac{20}{21}={{6 \times 20} \over {25 \times 21}}=\frac{3\times 2 \times 4 \times5}{5 \times 5 \times 7 \times3}\)\(=\frac{8}{35}\) ;

c) \( \frac{40}{7} \times \frac{14}{5} =\frac{40 \times 14}{7\times 5}=\frac{4\times 5 \times 2 \times 2 \times 7}{5\times7}=16\) ;

d) \( \frac{17}{13}:\frac{51}{26}= \frac{17}{13}\times \frac{26}{51}={{17 \times 26} \over {13 \times 51}}=\frac{17 \times 2 \times 13}{13 \times 3 \times 17}\)\(=\frac{2}{3}\) .