Bài 2 trang 25 Toán 7 Tập 1 | Cánh diều Giải Toán lớp 7

Giải Toán lớp 7 Bài 4: Thứ tự thực hiện các phép tính. Quy tắc dấu ngoặc

Bài 2 trang 25 Toán lớp 7 Tập 1: Tính:

a) $\left(\frac{4}{5}-1\right):\frac{3}{5}-\frac{2}{3}.\mathrm{0,5}$;

b) $1-{\left(\frac{5}{9}-\frac{2}{3}\right)}^2:\frac{4}{27}$;

c) $\left.\left(\frac{3}{8}-\frac{5}{12}\right).6+\frac{1}{3}\right].4$;

d) $\mathrm{0,8}:\left.\mathrm{0,2}-7.\left.\frac{1}{6}+\left(\frac{5}{21}-\frac{5}{14}\right)\right]\right\}$.

Lời giải:

Lời giải:

a) $\left(\frac{4}{5}-1\right):\frac{3}{5}-\frac{2}{3}.\mathrm{0,5}$

$=\left(\frac{-1}{5}\right):\frac{3}{5}-\frac{2}{3}.\frac{1}{2}$

$=\frac{-1}{5}.\frac{5}{3}-\frac{2}{3}.\frac{1}{2}$

$=\frac{-1}{3}-\frac{1}{3}$

$=-\left(\frac{1}{3}+\frac{1}{3}\right)=-\frac{2}{3}$.

b) $1-{\left(\frac{5}{9}-\frac{2}{3}\right)}^2:\frac{4}{27}$ 

$=1-{\left(\frac{5}{9}-\frac{6}{9}\right)}^2:\frac{4}{27}$

$=1-{\left(\frac{-1}{9}\right)}^2.\frac{27}{4}$

$=1-\frac{{(-1)}^2}{9^2}.\frac{27}{4}$

$=1-\frac{1}{81}.\frac{27}{4}$

$=1-\frac{1}{27.3}.\frac{27}{4}$

$=1-\frac{1}{12}=\frac{11}{12}$.

c)  $\left.\left(\frac{3}{8}-\frac{5}{12}\right).6+\frac{1}{3}\right].4$

$=\left.\left(\frac{9}{24}-\frac{10}{24}\right).6+\frac{1}{3}\right].4$

$=\left(\frac{-1}{24}.6+\frac{1}{3}\right).4$

$=\left(\frac{-1}{4}+\frac{1}{3}\right).4$

$=\left(\frac{-3}{12}+\frac{4}{12}\right).4$

$=\frac{1}{12}.4=\frac{1}{3}$.

d) $\mathrm{0,8}:\left.\mathrm{0,2}-7.\left.\frac{1}{6}+\left(\frac{5}{21}-\frac{5}{14}\right)\right]\right\}$

$=\frac{4}{5}:\left.\frac{1}{5}-7.\left.\frac{1}{6}+\left(\frac{10}{42}-\frac{15}{42}\right)\right]\right\}$

$=\frac{4}{5}:\left.\frac{1}{5}-7.\left.\frac{1}{6}+\frac{-5}{42}\right]\right\}$

$=\frac{4}{5}:\left.\frac{1}{5}-7.\left.\frac{7}{42}+\frac{-5}{42}\right]\right\}$

$=\frac{4}{5}:\left.\frac{1}{5}-7.\frac{2}{42}\right\}$

$=\frac{4}{5}:\left.\frac{1}{5}-7.\frac{2}{\mathrm{2.3.7}}\right\}$

$=\frac{4}{5}:\left.\frac{1}{5}-\frac{1}{3}\right\}$

$=\frac{4}{5}:\left.\frac{3}{15}-\frac{5}{15}\right\}$

$=\frac{4}{5}:\frac{-2}{15}$

$=\frac{4}{5}.\frac{-15}{2}=-6$.