[tex]\it x^2+7x-8=0,\ a=1,\ b=7,\ c=-8\\ \\ \Delta=b^2-4ac=7^2-4\cdot1\cdot(-8)=49+32=81\\ \\ x_{1,2}=\dfrac{-b\pm\sqrt{\Delta}}{2a}=\dfrac{-7\pm\sqrt{81}}{2\cdot1}=\dfrac{-7\pm9}{2} \Rightarrow \begin{cases} \it x_1=\dfrac{-7-9}{2}=\dfrac{-16}{2}=-8\\ \\ \\ \it x_2=\dfrac{-7+9}{2}=\dfrac{2}{2}=1\end{cases}[/tex]
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[tex]\it x^2-36=0 \Rightarrow x^2=36 \Rightarrow \sqrt{x^2}=\sqrt{36} \Rightarrow |x|=6 \Rightarrow x=\pm6[/tex]
