\(\Leftrightarrow\left(x-3\right)\left(2x^2+2\right)+5x\left(3-x\right)=0\)
\(\Leftrightarrow\left(x-3\right)\left(2x^2-5x+2\right)=0\)
=>(x-3)(x-2)(2x-1)=0
=>x=3 hoặc x=2 hoặc x=1/2
\(2\left(x-3\right)\left(x^2+1\right)+15x-5x^2=0\\ \Leftrightarrow\left(x-3\right)\left(2x^2+2\right)-5x\left(x-3\right)=0\\ \Leftrightarrow\left(x-3\right)\left(2x^2-5x+2\right)=0\\ \Leftrightarrow\left(x-3\right)\left[\left(2x^2-4x\right)-\left(x-2\right)\right]=0\\ \Leftrightarrow\left(x-3\right)\left[2x\left(x-2\right)-\left(x-2\right)\right]=0\\ \Leftrightarrow\left(x-3\right)\left(x-2\right)\left(2x-1\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=3\\x=2\\x=\dfrac{1}{2}\end{matrix}\right.\)
\(2\left(x-3\right)\left(x^2+1\right)+15x-5x^2=0\)
\(\Leftrightarrow2x^3+2x-6x^2-6+15x-5x^2=0\)
\(\Leftrightarrow2x^3-11x^2+17x-6=0\)
\(\Leftrightarrow2x^3-4x^2-7x^2+14x+3x-6=0\)
\(\Leftrightarrow2x^2\left(x-2\right)-7x\left(x-2\right)+3\left(x-2\right)=0\)
\(\Leftrightarrow\left(x-2\right)\left(2x^2-7x+3\right)=0\)
\(\Leftrightarrow\left(x-2\right)\left(2x^2-x-6x+3\right)=0\)
\(\Leftrightarrow\left(x-2\right)\left[x\left(2x-1\right)-3\left(2x-1\right)\right]=0\)
\(\Leftrightarrow\left(x-2\right)\left(2x-1\right)\left(x-3\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x-2=0\\2x-1=0\\x-3=0\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=2\\x=\dfrac{1}{2}\\x=3\end{matrix}\right.\)
Vậy: \(S=\left\{2;\dfrac{1}{2};3\right\}\)
