[tex]\it a)\ \ x^2=81 \Rightarrow \sqrt{x^2}=\sqrt{81} \Rightarrow |x|=9 \Rightarrow x=\pm9 \Rightarrow x\in\{-9,\ \ 9 \}\\ \\ \\ b)\ x^2=\dfrac{4}{49} \Rightarrow x=\pm\dfrac{2}{7} \Rightarrow x\in\Big\{-\dfrac{2}{7},\ \dfrac{2}{7}\Big\}\\ \\ \\ c)\ x^2=\ -25<0 \Rightarrow x\in\varnothing,\ ecua\c{\it t}ia\ este\ imposibil\breve a\ pentru\ \forall\ x\in\mathbb{Q}[/tex]
