a: \(\Leftrightarrow x\left(81x^2-16\right)=0\)
\(\Rightarrow x\left(9x-4\right)\left(9x+4\right)=0\)
hay \(x\in\left\{0;\dfrac{4}{9};-\dfrac{4}{9}\right\}\)
b: =>(x+7)(x-5)=0
=>x=5 hoặc x=-7
c: \(\Leftrightarrow7x^2-35x+x-5=0\)
=>(x-5)(7x+1)=0
=>x=-1/7 hoặc x=5
`a)`\(81x^3-16x=0\)
\(\Leftrightarrow x\left(81x^2-16\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\81x^2=16\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=\pm\dfrac{4}{9}\end{matrix}\right.\)
Vậy \(S=\left\{0;\pm\dfrac{4}{9}\right\}\)
`b)`\(x^2+2x-35=0\)
\(\Leftrightarrow x^2+2x+1-36=0\)
\(\Leftrightarrow\left(x+1\right)^2=36\)
\(\Leftrightarrow\left[{}\begin{matrix}x+1=6\\x+1=-6\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=5\\x=-7\end{matrix}\right.\)
Vậy \(S=\left\{5;-7\right\}\)
`c)`\(7x^2-34x-5=0\)
\(\Leftrightarrow7x^2-35x+x-5=0\)
\(\Leftrightarrow7x\left(x-5\right)+\left(x-5\right)=0\)
\(\Leftrightarrow\left(x-5\right)\left(7x+1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=5\\x=-\dfrac{1}{7}\end{matrix}\right.\)
Vậy \(S=\left\{5;-\dfrac{1}{7}\right\}\)
