a) \(\sqrt{x}=9\) (ĐK: \(x\ge0\))
\(\Leftrightarrow x=9^2\)
\(\Leftrightarrow x=81\left(tm\right)\)
b) \(\sqrt{x-2}=11\) (ĐK: \(x\ge2\))
\(\Leftrightarrow x-2=11^2\)
\(\Leftrightarrow x-2=121\)
\(\Leftrightarrow x=121+2=123\left(tm\right)\)
c) \(3\sqrt{x}-18=3\) (ĐK: \(x\ge0\))
\(\Leftrightarrow3\sqrt{x}=3+18\)
\(\Leftrightarrow3\sqrt{x}=21\)
\(\Leftrightarrow\sqrt{x}=\dfrac{21}{3}\)
\(\Leftrightarrow\sqrt{x}=7\)
\(\Leftrightarrow x=7^2\)
\(\Leftrightarrow x=49\left(tm\right)\)
d) \(2-5\sqrt{x}=-13\)
\(\Leftrightarrow5\sqrt{x}=2+13\)
\(\Leftrightarrow5\sqrt{x}=15\)
\(\Leftrightarrow\sqrt{x}=\dfrac{15}{5}\)
\(\Leftrightarrow\sqrt{x}=3\)
\(\Leftrightarrow x=3^2=9\left(tm\right)\)