Giải bài 5 trang 27 SGK Toán 7 tập 1 – Chân trời sáng tạo

Đề bài

Tìm x, biết:

a)\( – \frac{3}{5}.x = \frac{{12}}{{25}};\)

b)\(\frac{3}{5}x – \frac{3}{4} =  – 1\frac{1}{2};\)

c)\(\frac{2}{5} + \frac{3}{5}:x = 0,5;\)

d)\(\frac{3}{4} – \left( {x – \frac{1}{2}} \right) = 1\frac{2}{3}\)

e)\(2\frac{2}{{15}}:\left( {\frac{1}{3} – 5x} \right) =  – 2\frac{2}{5}\)

g)\({x^2} + \frac{1}{9} = \frac{5}{3}:3.\)

Lời giải chi tiết

a)

\(\begin{array}{l} – \frac{3}{5}.x = \frac{{12}}{{25}}\\x = \frac{{12}}{{25}}:\frac{{ – 3}}{5}\\x = \frac{{12}}{{25}}.\frac{{ – 5}}{3}\\x = \frac{{ – 4}}{5}\end{array}\)

Vậy \(x = \frac{{ – 4}}{5}\)

b)

\(\begin{array}{l}\frac{3}{5}x – \frac{3}{4} =  – 1\frac{1}{2};\\\frac{3}{5}x = \frac{{ – 3}}{2} + \frac{3}{4}\\\frac{3}{5}x = \frac{{ – 3}}{4}\\x = \frac{{ – 3}}{4}:\frac{3}{5}\\x = \frac{{ – 3}}{4}.\frac{5}{3}\\x = \frac{{ – 5}}{4}\end{array}\)

Vậy \(x = \frac{{ – 5}}{4}\).

c)

\(\begin{array}{l}\frac{2}{5} + \frac{3}{5}:x = 0,5\\\frac{3}{5}:x = \frac{1}{2} – \frac{2}{5}\\\frac{3}{5}:x = \frac{1}{{10}}\\x = \frac{3}{5}:\frac{1}{{10}}\\x = \frac{3}{5}.10\\x = 6\end{array}\)

Vậy \(x = 6\).

d)

\(\begin{array}{l}\frac{3}{4} – \left( {x – \frac{1}{2}} \right) = 1\frac{2}{3}\\x – \frac{1}{2} = \frac{3}{4} – \frac{5}{3}\\x – \frac{1}{2} = \frac{{ – 11}}{{12}}\\x = \frac{{ – 11}}{{12}} + \frac{1}{2}\\x = \frac{{ – 5}}{{12}}\end{array}\)

Vậy \(x = \frac{{ – 5}}{{12}}\).

e)

\(\begin{array}{l}2\frac{2}{{15}}:\left( {\frac{1}{3} – 5x} \right) =  – 2\frac{2}{5}\\\frac{{32}}{{15}}:\left( {\frac{1}{3} – 5x} \right) =  – \frac{{12}}{5}\\\frac{1}{3} – 5x = \frac{{32}}{{15}}:\frac{{ – 12}}{5}\\\frac{1}{3} – 5x = \frac{{32}}{{15}}.\frac{{ – 5}}{12}\\\frac{1}{3} – 5x = \frac{{ – 8}}{9}\\5x = \frac{1}{3} + \frac{8}{9}\\5x = \frac{{11}}{9}\\x = \frac{{11}}{9}:5\\x = \frac{{11}}{{45}}\end{array}\)

Vậy \(x = \frac{{11}}{{45}}\).

g)

\({x^2} + \frac{1}{9} = \frac{5}{3}:3\\{x^2} + \frac{1}{9} = \frac{5}{9}\\{x^2} = \frac{5}{9} – \frac{1}{9}\\{x^2} = \frac{4}{9}\\{x^2} = (\frac{2}{3})^2\\x = \frac{2}{3}\,\ hoặc \,\ x = \frac{-2}{3}\)

Vậy \(x \in \{\frac{2}{3};\frac{-2}{3}\}\).