Giải bài 2 trang 27 SGK Toán 7 tập 1 - Chân trời sáng tạo

Đề bài

Thực hiện phép tính (bằng cách hợp lí nếu có thể).

a)\(\frac{5}{{23}} + \frac{7}{{17}} + 0,25 - \frac{5}{{23}} + \frac{{10}}{{17}}\)

b)\(\frac{3}{7}.2\frac{2}{3} - \frac{3}{7}.1\frac{1}{2};\)

c)\(13\frac{1}{4}:\left( { - \frac{4}{7}} \right) - 17\frac{1}{4}:\left( { - \frac{4}{7}} \right);\)

d)\(\frac{{100}}{{123}}:\left( {\frac{3}{4} + \frac{7}{{12}}} \right) + \frac{{23}}{{123}}:\left( {\frac{9}{5} - \frac{7}{{15}}} \right).\)

Lời giải chi tiết

a)

\(\begin{array}{l}\frac{5}{{23}} + \frac{7}{{17}} + 0,25 - \frac{5}{{23}} + \frac{{10}}{{17}}\\ = \left( {\frac{5}{{23}} - \frac{5}{{23}}} \right) + \left( {\frac{7}{{17}} + \frac{{10}}{{17}}} \right) + 0,25\\ = 0 + \frac{{17}}{{17}} + \frac{{25}}{{100}}\\ = 1 + \frac{1}{4}\\ = \frac{5}{4}\end{array}\)

b)

\(\begin{array}{l}\frac{3}{7}.2\frac{2}{3} - \frac{3}{7}.1\frac{1}{2}\\ = \frac{3}{7}.\frac{8}{3} - \frac{3}{7}.\frac{3}{2}\\ = \frac{3}{7}.\left( {\frac{8}{3} - \frac{3}{2}} \right)\\ = \frac{3}{7}.\left( {\frac{{16}}{6} - \frac{9}{6}} \right)\\ = \frac{3}{7}.\frac{7}{6}\\ = \frac{1}{2}\end{array}\)

c)

\(\begin{array}{l}13\frac{1}{4}:\left( { - \frac{4}{7}} \right) - 17\frac{1}{4}:\left( { - \frac{4}{7}} \right)\\ = 13\frac{1}{4}.\frac{{ - 7}}{4} - 17\frac{1}{4}.\frac{{ - 7}}{4}\\ = \frac{{ - 7}}{4}.\left( {13\frac{1}{4} - 17\frac{1}{4}} \right)\\ = \frac{{ - 7}}{4}.\left( { - 4} \right)\\ = 7\end{array}\)

d)

\(\begin{array}{l}\frac{{100}}{{123}}:\left( {\frac{3}{4} + \frac{7}{{12}}} \right) + \frac{{23}}{{123}}:\left( {\frac{9}{5} - \frac{7}{{15}}} \right)\\ = \frac{{100}}{{123}}:\left( {\frac{9}{{12}} + \frac{7}{{12}}} \right) + \frac{{23}}{{123}}:\left( {\frac{{27}}{{15}} - \frac{7}{{15}}} \right)\\ = \frac{{100}}{{123}}:\frac{{16}}{{12}} + \frac{{23}}{{123}}:\frac{{20}}{{15}}\\ = \frac{{100}}{{123}}:\frac{4}{3} + \frac{{23}}{{123}}:\frac{4}{3}\\ = \frac{{100}}{{123}}.\frac{3}{4} + \frac{{23}}{{123}}.\frac{3}{4}\\ = \frac{3}{4}.\left( {\frac{{100}}{{123}} + \frac{{23}}{{123}}} \right)\\ = \frac{3}{4}.\frac{{123}}{{123}}\\ = \frac{3}{4}.1\\ = \frac{3}{4}\end{array}\)