ko nói linh tinh bạn nhé

\(a,\dfrac{5}{9}\times\dfrac{5}{2}+\dfrac{4}{9}\times\dfrac{5}{2}=\dfrac{5}{2}\times\left(\dfrac{5}{9}+\dfrac{4}{9}\right)=\dfrac{5}{2}\times1=\dfrac{5}{2}\)

\(12\times\dfrac{3}{4}-\dfrac{3}{4}\times\dfrac{12}{7}=\dfrac{3}{4}\times\left(12-\dfrac{12}{7}\right)=\dfrac{3}{4}\times\dfrac{72}{7}=\dfrac{54}{7}\)

\(x-\dfrac{2}{3}=-\dfrac{5}{12}\)

\(\Rightarrow x=-\dfrac{5}{12}+\dfrac{2}{3}\)

\(\Rightarrow x=-\dfrac{5}{12}+\dfrac{8}{12}\)

\(\Rightarrow x=\dfrac{3}{12}=\dfrac{1}{4}\)

`----`

\(\dfrac{x}{15}=\dfrac{3}{4}+\dfrac{19}{-20}\)

\(\Rightarrow\dfrac{x}{15}=-\dfrac{1}{5}\)

\(\Rightarrow-\dfrac{3}{15}=-\dfrac{1}{5}\)

\(\Rightarrow x=-3\)

`-----`

\(\dfrac{x}{-2}=-\dfrac{8}{x}\)

\(\Rightarrow x^2=-2.\left(-8\right)\)

\(\Rightarrow x^2=16\)

\(\Rightarrow\left[{}\begin{matrix}x=4\\x=-4\end{matrix}\right.\)

\(\dfrac{5}{x-2}=\dfrac{10}{16}\)

\(\Rightarrow5.16=\left(x-2\right).10\)

\(\Rightarrow\left(x-2\right).10=80\)

\(\Rightarrow x-2=80:10\)

\(\Rightarrow x-2=8\)

\(\Rightarrow x=8+2\)

\(\Rightarrow x=10\)

\(\left(x^2-4\right).\left(x^2-3\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x^2-4=0\\x^2-3=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x^2=4\\x^2=3\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=\pm2\\x=\pm\sqrt{3}\end{matrix}\right.\)

\(\text{51.39}\)

\(=\)\(\text{51.(40-1)}\)

\(\text{=51.40-51.1}\)

\(\text{=2040-51}\)

\(\text{=1989}\)

\(\text{|5x|=x-12}\)

ĐKXĐ : \(x\ge12\)

\(=>\left[{}\begin{matrix}\text{5x = x − 12}\\\text{5x = 12 − x }\end{matrix}\right.=>\left[{}\begin{matrix}\text{x = 2}\\\text{x = − 3}\end{matrix}\right.\) \(\text{(loại)}\)

\(\text{(x-3) ×4=12}\)

\(\text{x-3=12:4=3}\)

\(\text{x=3+3=6}\)