`a)` `x^2(x-5)-9x=45`
`<=>x^3-5x^2-9x-45=0`
`<=>x^2(x-5)-9(x-5)=0`
`<=>(x-5)(x^2-9)=0`
`<=>(x-5)(x-3)(x+3)=0`
`<=>`$\left[\begin{matrix} x-5=0\\ x-3=0\\ x+3=0\end{matrix}\right.$
`<=>`$\left[\begin{matrix} x=5\\ x=3\\ x=-3\end{matrix}\right.$
`text{vậy}` `S={5;3;-3}`
`b)` `9(5-x)+x^2-10x=-25`
`<=>x^2-10x+25-9(x-5)=0`
`<=>(x-5)^2-9(x-5)=0`
`<=>(x-5)(x-5-9)=0`
`<=>(x-5)(x-14)=0`
`<=>`$\left[\begin{matrix} x-5=0\\ x-14=0\end{matrix}\right.$
`<=>`$\left[\begin{matrix} x=5\\ x=14\end{matrix}\right.$
`text{vậy}` `S={5;14}`
`text{xin 5 sao và hay nhất nha}`
