`#Sad`
`b)`
`f(x) = 2x^2+3`
`2x^2+3 = 0`
`⇔ 2x^2 = -3`
`⇔ x^2 = -3/2`
`\text{→Vì}` `x^2 >= 0`
`\text{→Mà}` `x^2 = -3/2` `\text{(vô lý)}`
`\text{Vậy}` `x` `\text{vô nghiệm}`
`c)`
`f(x) = x^2+5x`
`x^2+5x = 0`
`⇔ x(x+5) = 0`
`⇔` \(\left[ \begin{array}{l}x=0\\x+5=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=0\\x=-5\end{array} \right.\)
`\text{Vậy S=}` `{0; -5}`
`d)`
`f(x) = -(1)/(2)x+(3)/(4)x+1`
`-(1)/(2)x+(3)/(4)x+1 = 0`
`⇔ (1)/(4)x = -1`
`⇔ x = -4`
`\text{Vậy S=}` `{-4}`
`e)`
`f(x) = x^2-1/4`
`x^2-1/4 = 0`
`⇔ x^2 = 1/4`
`⇔` \(\left[ \begin{array}{l}x=\dfrac{1}{2}\\x=-\dfrac{1}{2}\end{array} \right.\)
`\text{Vậy S=}` `{1/2; -1/2}`
`f)`
`f(x) = x^2+3x+2`
`x^2+3x+2 = 0`
`⇔ x^2+2x+x+2 = 0`
`⇔ (x^2+2x)+(x+2) = 0`
`⇔ x(x+2)+(x+2) = 0`
`⇔ (x+1)(x+2) = 0`
`⇔` \(\left[ \begin{array}{l}x+1=0\\x+2=0\end{array} \right.\)
`⇔` \(\left[ \begin{array}{l}x=-1\\x=-2\end{array} \right.\)
`\text{Vậy S=}` `{-1; -2}`
