Đáp án:
$\text{ a) x² - 100 =0}$
$\text{=> x -10 =0}$
$\text{=> x =10 ; -10}$
$\text{Vậy x ∈{10;-10} }$
$\text{b) 4x² - 81 =0}$
$\text{=> 2x - 9 = 0 }$
$\text{=>x = $\dfrac{9}{2}$ ; $\dfrac{-9}{2}$ }$
$\text{Vậy x ∈ {$\dfrac{9}{2}$ ; $\dfrac{-9}{2}$ } }$
$\text{c) x² - 7 = 0}$
$\text{=> x - √7 =0 }$
$\text{=> x = √7 ; √-7 }$
$\text{Vậy x ∈ { √7 ; √-7 } }$
$\text{d) x² - 9x = 0 }$
$\text{=> x(x-9)=0}$
$\text{=> \(\left[ \begin{array}{l}x=0\\x-9=0\end{array} \right.\) }$
$\text{=> \(\left[ \begin{array}{l}x=0\\x=9\end{array} \right.\) }$
$\text{Vậy x ∈ {0 ;9} }$
$\text{h) x³ + 3x =0 }$
$\text{=> x (x²+3)=0 }$
$\text{=> \(\left[ \begin{array}{l}x=0\\x^2+3=0\end{array} \right.\) }$
$\text{=> \(\left[ \begin{array}{l}x=0\\x=x^2+3 (loại)\end{array} \right.\) }$
$\text{Vậy x ∈ { 0 } }$
