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`x^2-2018x-2019=0`
`⇔ x^2-2019x+x-2019=0`
`⇔ (x^2-2019x)+(x-2019)=0`
`⇔ x(x-2019)+(x-2019)=0`
`⇔ (x-2019)(x+1)=0`
`⇔` \(\left[ \begin{array}{l}x-2019=0\\x+1=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=2019\\x=-1\end{array} \right.\)
Vậy `S={2019; -1}`
`x^2-x+1/4=0`
`⇔ x^2+2.(1)/2x+(1/2)^2=0`
`⇔ (x+1/2)^2=0`
`⇔ x+1/2=0`
`⇔ x=(-1)/2`
Vậy `S={(-1)/2}`
`4x^4-101x^2+25=0`
`⇔ (4x^4-20x^2+25)-81x^2=0`
`⇔ [(2x^2)^2-2.2x^(2).5+5^2]-81x^2=0`
`⇔ (2x^2-5)^2-81x^2=0`
`⇔ (2x^2-5)^2-(9x)^2=0`
`⇔ (2x^2-5-9x)(2x^2-5+9x)=0`
`⇔ [(2x^2-50)-(9x-45)][(2x^2-50)+(9x+45)]=0`
`⇔ [2(x-5)(x+5)-9(x-5)][2(x-5)(x+5)+9(x+5)]=0`
`⇔ (x-5)[2(x+5)-9](x+5)[2(x-5)+9]=0`
`⇔ (x-5)(2x+10-9)(x+5)(2x-10+9)=0`
`⇔ (x-5)(2x+1)(x+5)(2x-1)=0`
`⇔` \(\left[ \begin{array}{l}x-5=0\\2x+1=0\\x+5=0\\2x-1=0\end{array} \right.\)
`⇔ ` \(\left[ \begin{array}{l}x=5\\x=-1/2\\x=-5\\x=1/2\end{array} \right.\)
Vậy `S={5; -5; (-1)/2; 1/2}`
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