Giải bài 2 trang 15

Đề bài

Tính:

a)$0,75-\frac{5}{6}+1\frac{1}{2};$                                    

b)$\frac{3}{7}+\frac{4}{15}+\left(\frac{-8}{21}\right)+\left(-0,4\right);$

c)$0,625+\left(\frac{-2}{7}\right)+\frac{3}{8}+\left(\frac{-5}{7}\right)+1\frac{2}{3}$           

d)$\left(-3\right).\left(\frac{-38}{21}\right).\left(\frac{-7}{6}\right).\left(-\frac{3}{19}\right);$

e) $\left(\frac{11}{18}:\frac{22}{9}\right).\frac{8}{5};$                                    

g)$\left[\left(\frac{-4}{5}\right).\frac{5}{8}\right]:\left(\frac{-25}{12}\right)$

Lời giải chi tiết

a)

$\begin{array}{c}0,75-\frac{5}{6}+1\frac{1}{2}=\frac{3}{4}-\frac{5}{6}+\frac{3}{2}\\ =\frac{9}{12}-\frac{10}{12}+\frac{18}{12}=\frac{17}{12}\end{array}$                                    

b)

$\begin{array}{c}\frac{3}{7}+\frac{4}{15}+\left(\frac{-8}{21}\right)+\left(-0,4\right)=\frac{3}{7}+\frac{4}{15}-\frac{8}{21}-\frac{2}{5}\\ =\left(\frac{3}{7}-\frac{8}{21}\right)+\left(\frac{4}{15}-\frac{2}{5}\right)\\ =\left(\frac{9}{21}-\frac{8}{21}\right)+\left(\frac{4}{15}-\frac{6}{15}\right)\\ =\frac{1}{21}+\left(\frac{-2}{15}\right)\\ =\frac{5}{105}-\frac{14}{105}\\ =\frac{-9}{105}=\frac{-3}{35}\end{array}$

c)

$\begin{array}{c}0,625+\left(\frac{-2}{7}\right)+\frac{3}{8}+\left(\frac{-5}{7}\right)+1\frac{2}{3}\\ =\frac{5}{8}+\left(\frac{-2}{7}\right)+\frac{3}{8}-\frac{5}{7}+\frac{5}{3}\\ =\left(\frac{5}{8}+\frac{3}{8}\right)+\left(\frac{-2}{7}-\frac{5}{7}\right)+\frac{5}{3}\\ =1-1+\frac{5}{3}=\frac{5}{3}\end{array}$          

 d)

$\begin{array}{c}\left(-3\right).\left(\frac{-38}{21}\right).\left(\frac{-7}{6}\right).\left(-\frac{3}{19}\right)\\ =\frac{-3.\left(-38\right).\left(-7\right).\left(-3\right)}{\mathrm{21.6.19}}\\ =\frac{\mathrm{3.38.7.3}}{\mathrm{21.6.19}}\\ =\frac{\mathrm{3.2.19.7.3}}{\mathrm{3.7.3.2.19}}\\ =1\end{array}$

e)

 $\begin{array}{c}\left(\frac{11}{18}:\frac{22}{9}\right).\frac{8}{5}=\left(\frac{11}{18}.\frac{9}{22}\right).\frac{8}{5}\\ =\frac{\mathrm{11.9.4.2}}{\mathrm{9.2.2.11.5}}=\frac{2}{5}\end{array}$                                   

 g)

$\left[\left(\frac{-4}{5}\right).\frac{5}{8}\right]:\left(\frac{-25}{12}\right)=\frac{-20}{40}:\left(\frac{-25}{12}\right)=\frac{-1}{2}.\frac{-12}{25}=\frac{6}{25}$