`\text{~~Holi~~}`
`x^2+\sqrt{x^2-4x-5}=4x+25`
`-> \sqrt{x^2-4x-5}=4x+25-x^2`
`-> x^2-4x-5=16x^2+625+x^4+200x-8x^3-50x^2`
`-> x^2-4x-5=-34x^2+625+x^4+200x-8x^3`
`-> x^2-4x-5+34x^2-625-x^4-200x+8x^3=0`
`-> 35x^2-204x-630-x^4+8x^3=0`
`-> -x^4+8x^2+35x^2-204x-630=0`
`-> -x^4-3x^3+11x^3+33x^2+2x^2+6x-210x-630=0`
`-> -x^3(x+3)+11x^2(x+3)+2x(x+3)-210(x+3)=0`
`-> -(x+3)(x^3-11x^2-2x+210)=0`
`-> -(x+3)(x^2-7x^2-4x^2+28x-30x+210)=0`
`-> -(x+3)[x^2(x-7)-4x(x-7)-30(x-7)]=0`
`-> -(x+3)(x-7)(x^2-4x-30)=0`
`-> (x+3)(x-7)(x^2-4x-30)=0`
`->`\(\left[ \begin{array}{l}x+3=0\\x-7=0\\x^2-4x-30=0\end{array} \right.\)
`->`\(\left[ \begin{array}{l}x=-3\\x=7\\x=2+\sqrt{34} (\text{không phải là nghiệm})\\x=2-\sqrt{34}(\text{không phải là nghiệm})\end{array} \right.\)
`->`\(\left[ \begin{array}{l}x=-3\\x=7\end{array} \right.\)
Vậy `S={-3,7}`
