Giải bài 13 trang 12 - SGK Toán lớp 7 Tập 1

Hướng dẫn:   \(\dfrac{a}{b}. \dfrac{c}{d} = \dfrac{a.c}{b.d}\)

Bài giải

a) \( \dfrac{-3}{4}.\dfrac{12}{-5}.\left(-\dfrac{25}{6}\right)\)
\(= \dfrac{-3.(-12).(-25)}{4.5.6}\)
\(= \dfrac{-15}{2} = -7\dfrac{1}{2}\)
b) \( (-2).\dfrac{-38}{21}.\dfrac{-7}{4}.\left(-\dfrac{3}{8}\right)\)
\(= \dfrac{(-2).(-38).(-7).(-3)}{21.4.8}\)
\(= \dfrac{19}{8} = 2\dfrac{3}{8}\)
c) \(\left(\dfrac{11}{12}:\dfrac{33}{16}\right).\dfrac{3}{5}\)
\(= \left(\dfrac{11}{12}.\dfrac{16}{33}\right).\dfrac{3}{5}\)
\(= \dfrac{11.16.3}{12.33.5}\)
\(= \dfrac{4}{15}\)
d) \(\dfrac{7}{23}.\left[\left(-\dfrac{8}{6}\right)-\dfrac{45}{18}\right]\)
\(= \dfrac{7}{23}.\dfrac{-24 - 25}{18}\)
\(= \dfrac{7}{23}.\left(\dfrac{-69}{18}\right)\)
\(= \dfrac{7.(-69)}{23.18}\)
\(= -\dfrac{7}{6}\)
\(= -1\dfrac{1}{6}\)