a) \(\frac{3}{4}+\frac{1}{4}:x=\frac{2}{5}\)
\(\Rightarrow\frac{1}{4}:x=\frac{2}{5}-\frac{3}{4}\)
\(\Rightarrow\frac{1}{4}:x=-\frac{7}{20}\)
\(\Rightarrow x=\frac{1}{4}:\left(-\frac{7}{20}\right)\)
\(\Rightarrow x=-\frac{5}{7}\)
Vậy \(x=-\frac{5}{7}.\)
b) \(\frac{x}{-2}=\frac{-8}{x}\)
\(\Rightarrow x.x=\left(-8\right).\left(-2\right)\)
\(\Rightarrow x^2=16\)
\(\Rightarrow\left[{}\begin{matrix}x=4\\x=-4\end{matrix}\right.\)
Vậy \(x\in\left\{4;-4\right\}.\)
c) \(\frac{x-1}{5}=\frac{20}{x-1}\)
\(\Rightarrow\left(x-1\right).\left(x-1\right)=20.5\)
\(\Rightarrow\left(x-1\right).\left(x-1\right)=100\)
\(\Rightarrow\left(x-1\right)^2=100\)
\(\Rightarrow x-1=\pm10.\)
\(\Rightarrow\left[{}\begin{matrix}x-1=10\\x-1=-10\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=10+1\\x=\left(-10\right)+1\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=11\\x=-9\end{matrix}\right.\)
Vậy \(x\in\left\{11;-9\right\}.\)
Chúc bạn học tốt!
