$1/a)2\sqrt[]{80}-4\sqrt[]{45}+6\sqrt[]{20}-7\sqrt[]{5}$
$=2\sqrt[]{4^2.5}-4\sqrt[]{3^2.5}+6.\sqrt[]{2^2.5}-7\sqrt[]{5}$
$=8\sqrt[]{5}-12\sqrt[]{5}+12\sqrt[]{5}-7\sqrt[]{5}$
$=\sqrt[]{5}$
$b)\dfrac{2}{\sqrt[]{5}-2}+$ $\dfrac{3}{\sqrt[]{5}+2}$
$=\dfrac{2(\sqrt[]{5}+2)+3(\sqrt[]{5}-2)}{(\sqrt[]{5}+2)(\sqrt[]{5}-2)}$
$=2\sqrt[]{5}+4+3\sqrt[]{5}-6$
$=5\sqrt[]{5}-2$
$2/a)\sqrt[]{2x-5}=3$ với $x≥\dfrac{5}{2}$
$⇔2x-5=9$
$⇔2x=14$
$⇔x=7$
$b)\sqrt[]{6x+1}=x+1$ với $x≥-1$
$⇔6x+1=x^2+2x+1$
$⇔x^2-4x=0$
$⇔x(x-4)=0$
$⇔$\(\left[ \begin{array}{l}x=0\\x-4=0\end{array} \right.\)
$⇔$\(\left[ \begin{array}{l}x=0\\x=4\end{array} \right.\)
$c)6\sqrt[]{x-3}+\sqrt[]{16x-48}-2\sqrt[]{9x-27}=12$
$⇔6\sqrt[]{x-3}+\sqrt[]{4^2(x-3)}-2\sqrt[]{3^2(x-3)}=12$
$⇔6\sqrt[]{x-3}+4\sqrt[]{x-3}-6\sqrt[]{x-3}=12$
$⇔4\sqrt[]{x-3}=12$
$⇔\sqrt[]{x-3}=3$ với $x≥3$
$⇔x-3=9$
$⇔x=12$
