Câu 1:
a) \(A=\dfrac{3}{\sqrt{7}-2}-\dfrac{14}{\sqrt{7}}+\sqrt{\left(\sqrt{7}-2\right)^2}=\dfrac{3\left(\sqrt{7}+2\right)}{7-4}-\dfrac{14\sqrt{7}}{7}+\sqrt{7}-2=\dfrac{3\sqrt{7}+6}{3}-2\sqrt{7}+\sqrt{7}-2=\sqrt{7}+2-2\sqrt{7}+\sqrt{7}-2=0\)
b) \(5x^2-2\sqrt{5}x+1=0\)
\(\Leftrightarrow\left(\sqrt{5}x-1\right)^2=0\)
\(\Leftrightarrow\sqrt{5}x-1=0\Leftrightarrow\sqrt{5}x=1\Leftrightarrow x=\dfrac{1}{\sqrt{5}}=\dfrac{\sqrt{5}}{5}\)
c) \(\left\{{}\begin{matrix}3x-2y=16\\x+5y=-23\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}3x-2y=16\\3x+15y=-69\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x+5y=-23\\-17y=85\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}x=2\\y=-5\end{matrix}\right.\)
