Đáp án:
$\rm S=\{\dfrac{{5 + \sqrt {505} }}{2}; \dfrac{{5 - \sqrt {505} }}{2}\}$
Giải thích các bước giải:
`x^2-5x-120=0`
`<=> x^2 - 2 . x . 5/2 + (5/2)^2 - 120 - (5/2)^2=0`
`<=> ( x - 5/2 )^2 - 505/4 = 0`
`<=> ( x - 5/2 )^2 = 505/4`
`<=>` \(\left[ \begin{array}{l}x - \dfrac{5}{2} = \dfrac{{\sqrt {505} }}{2}\\x - \dfrac{5}{2} = -\dfrac{{ \sqrt {505} }}{2}\end{array} \right.\) `<=>` \(\left[ \begin{array}{l}x = \dfrac{{5 + \sqrt {505} }}{2}\\ x = \dfrac{{5 - \sqrt {505} }}{2} \end{array} \right.\)
Vậy $\rm S=\{\dfrac{{5 + \sqrt {505} }}{2}; \dfrac{{5 - \sqrt {505} }}{2}\}$
