[tex]x \ \textgreater \ 0 \\ \\ x= \sqrt{81} \ \ \ | (...)^{2} \\ \\ x^{2}=81 \\ \\ x^{2}-81=0 \\ \\ (x-9)(x+9)=0 \\ \\ x-9=0 \ \vee \ x+9=0 \\ \\ \boxed{x=9} \ \vee \ x=-9 \not\in D[/tex]
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"Associate with people who are likely to improve you."
