Bài 4 trang 19

Đề bài

Tìm x, biết:

\(a)x - \dfrac{3}{4} = \dfrac{2}{7}\)

\(b) - \dfrac{3}{8}x - 0,75 =  - 1\dfrac{1}{2}\)

\(c)(0,25 - x):\dfrac{{ - 3}}{5} =  - \dfrac{3}{4}\)

\(d)\dfrac{{ - 3}}{5}.(4x - 1,2) =  - \dfrac{{12}}{{25}}\)

Lời giải chi tiết

\(\begin{array}{l}a)x - \dfrac{3}{4} = \dfrac{2}{7}\\ \Leftrightarrow x = \dfrac{2}{7} + \dfrac{3}{4}\\ \Leftrightarrow x = \dfrac{{8 + 21}}{{28}}\\ \Leftrightarrow x = \dfrac{{29}}{{28}}\end{array}\)

Vậy \(x = \dfrac{{29}}{{28}}\)

\(b) - \dfrac{3}{8}x - 0,75 =  - 1\dfrac{1}{2}\)

\( \Leftrightarrow  - \dfrac{3}{8}x - \dfrac{3}{4} =  - \dfrac{3}{2}\)

\(\begin{array}{l} \Leftrightarrow  - \dfrac{3}{8}x =  - \dfrac{3}{2} + \dfrac{3}{4}\\ \Leftrightarrow  - \dfrac{3}{8}x =  - \dfrac{6}{4} + \dfrac{3}{4} = \dfrac{{ - 3}}{4}\\ \Leftrightarrow  - \dfrac{3}{8}x =  - \dfrac{3}{4}\\ \Leftrightarrow x = \dfrac{3}{4}:\dfrac{3}{8} = 2\end{array}\)

Vậy \(x = 2\)

\(\begin{array}{l}c)(0,25 - x):\dfrac{{ - 3}}{5} =  - \dfrac{3}{4}\\ \Leftrightarrow \left( {\dfrac{1}{4} - x} \right).\dfrac{{ - 5}}{3} =  - \dfrac{3}{4}\\ \Leftrightarrow \left( {\dfrac{1}{4} - x} \right) =  - \dfrac{3}{4}:\dfrac{{ - 5}}{3}\\ \Leftrightarrow \left( {\dfrac{1}{4} - x} \right) =  - \dfrac{3}{4}.\dfrac{{ - 3}}{5}\\ \Leftrightarrow \dfrac{1}{4} - x = \dfrac{9}{{20}}\\ \Leftrightarrow x = \dfrac{1}{4} - \dfrac{9}{{20}} =  - \dfrac{1}{5}\end{array}\)

Vậy \(x =  - \dfrac{1}{5}\)

\(\begin{array}{l}d)\dfrac{{ - 3}}{5}.(4x - 1,2) =  - \dfrac{{12}}{{25}}\\ \Leftrightarrow \dfrac{{ - 3}}{5}.\left( {4x - \dfrac{6}{5}} \right) =  - \dfrac{{12}}{{25}}\\ \Leftrightarrow \left( {4x - \dfrac{6}{5}} \right) =  - \dfrac{{12}}{{25}}:\dfrac{{ - 3}}{5}\\ \Leftrightarrow 4x - \dfrac{6}{5} =  - \dfrac{{12}}{{25}}.\dfrac{{ - 5}}{3}\\ \Leftrightarrow 4x - \dfrac{6}{5} =  - \dfrac{{12}}{{25}}.\dfrac{{ - 5}}{3}\\ \Leftrightarrow 4x - \dfrac{6}{5} = \dfrac{4}{5}\\ \Leftrightarrow 4x = \dfrac{4}{5} + \dfrac{6}{5} = 2\\ \Leftrightarrow x = \dfrac{1}{2}\end{array}\)

Vậy \(x = \dfrac{1}{2}\)