tìm x x^2 - 2018x- 2019 = 0 x^2 - x+ 1/ 4 4x^4 -101x^2 + 25= 0

`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}`