Bài 13 trang 12 sgk Toán 7 tập 1, Tính -3/4 . 12/-5 (-25/6)....

Tính

a) $\frac{-3}{4}. \frac{12}{-5}. (\frac{-25}{6})$

b)  $(-2). \frac{-38}{21} .\frac{-7}{4} . (-\frac{3}{8})$

c) $(\frac{11}{12}: \frac{33}{16}).\frac{3}{5}$ 

d) $\frac{7}{23} . \left [ (-\frac{8}{6}) - \frac{45}{18} \right ]$

a)   $\frac{-3}{4}. \frac{12}{-5}. (\frac{-25}{6})$ = $\frac{-3}{4}. \frac{-12}{5} . \frac{-25}{6} = \frac{-3.(-12).(-25)}{4.5.6} = \frac{-3.5}{2} = -\frac{15}{2}$ 

b)   $(-2). \frac{-38}{21} .\frac{-7}{4} . (-\frac{3}{8})$ = $\frac{(-2)(-38)(-7)(-3)}{21.4.8} = \frac{38}{2.8} = \frac{19}{8} = 2\frac{3}{8}$     

c)  $(\frac{11}{12}: \frac{33}{16}).\frac{3}{5}$ = $(\frac{11}{12}.\frac{16}{33}).\frac{3}{5} = (\frac{4}{3.5}). \frac{3}{5} = \frac{4.3}{3.3.5} = \frac{4}{15}$   

d) $\frac{7}{23} . \left [ (-\frac{8}{6}) - \frac{45}{18}\ \right ]$ = $\frac{7}{23}.(\frac{-8}{6}- \frac{15}{6}) = \frac{-7}{6} = -1\frac{1}{6}$