`#Sad`
`a)`
`x/3+(x^2)/4 = 0`
`⇔ (4x)/(12)+(3x^2)/(12) = 0`
`⇔ 4x+3x^2 = 0`
`⇔ x(4+3x) = 0`
`⇔` \(\left[ \begin{array}{l}x=0\\4+3x=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=0\\3x=-4\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=0\\x=-\dfrac{4}{3}\end{array} \right.\)
`\text{Vậy S=}` `{0; -(4)/(3)}`
`b)`
`x+5 = 2(x+5)^2`
`⇔ (x+5)-2(x+5)^2 = 0`
`⇔ (x+5)[1-2(x+5)] = 0`
`⇔ (x+5)[(1-2x-10) = 0`
`⇔ (x+5)(-2x-9) = 0`
`⇔` \(\left[ \begin{array}{l}x+5=0\\-2x-9=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=-5\\-2x=9\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=-5\\x=-\dfrac{9}{2}\end{array} \right.\)
`\text{Vậy S=}` `{-5; -(9)/(2)}`
`c)`
`(x^2+1)(2x-1)+2x = 1`
`⇔ 2x^3-x^2+2x-1+2x-1 = 0`
`⇔ 2x^3-x^2+4x-2 = 0`
`⇔ (2x^3-x^2)+(4x-2) = 0`
`⇔ x^2(2x-1)+2(2x-1) = 0`
`⇔ (x^2+2)(2x-1) = 0`
`⇔` \(\left[ \begin{array}{l}x^2+2=0\\2x-1=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x^2==-2\\2x=1\end{array} \right.\)
`\text{Vì}` `x^2 >= 0` `AA x`
`\text{Mà}` `x^2 = -2` `\text{(vô lý)}`
`⇔ 2x = 1`
`⇔ x = 1/2`
`\text{Vậy S=}` `{1/2}`
