Tìm x, biết:
\(a)x – \dfrac{3}{4} = \dfrac{2}{7}\)
\(b) – \dfrac{3}{8}x – 0,75 = – 1\dfrac{1}{2}\)
\(c)(0,25 – x):\dfrac{{ – 3}}{5} = – \dfrac{3}{4}\)
\(d)\dfrac{{ – 3}}{5}.(4x – 1,2) = – \dfrac{{12}}{{25}}\)
Ta đổi các số thập phân thành phân số rồi sau đó tính toán tìm x theo thứ tự của phép tính .
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\(\begin{array}{l}a)x – \dfrac{3}{4} = \dfrac{2}{7}\\ \Leftrightarrow x = \dfrac{2}{7} + \dfrac{3}{4}\\ \Leftrightarrow x = \dfrac{{8 + 21}}{{28}}\\ \Leftrightarrow x = \dfrac{{29}}{{28}}\end{array}\)
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 \(x = \dfrac{1}{2}\)