12. \(A.M=\sqrt{a}\)
⇒ Chọn A
13. \(\dfrac{x-\sqrt{x}}{\sqrt{x}}=\dfrac{\sqrt{x}\left(\sqrt{x}-1\right)}{\sqrt{x}}=\sqrt{x}-1\)
⇒ Chọn D
14. \(\sqrt{11\dfrac{11}{120}}=\sqrt{\dfrac{1313}{120}}=\dfrac{11\sqrt{330}}{60}\)
⇒ Chọn C
15. \(3A=3\cdot\left(\dfrac{\sqrt{45}+\sqrt{5}}{\sqrt{125}-\sqrt{80}}\right)=3\cdot\left(\dfrac{3\sqrt{5}+\sqrt{5}}{5\sqrt{5}-4\sqrt{5}}\right)=3\cdot4=12\)
⇒ Chọn A
16. \(\left(\sqrt{5}+3\sqrt{2}\right)\left(2\sqrt{5}+\sqrt{2}\right)-\left(\sqrt{2}+3\sqrt{5}\right)\left(2\sqrt{2}+\sqrt{5}\right)=-3\)
⇒ Chọn D
17. \(N=\sqrt{9-4\sqrt{5}}+\sqrt{9+4\sqrt[]{5}}=\sqrt{\left(\sqrt{5}-2\right)^2}+\sqrt{\left(\sqrt{5}+2\right)^2}=2\sqrt{5}\)
⇒ Chọn D
18. \(3\sqrt{8a}+\dfrac{1}{4}\sqrt{\dfrac{32a}{25}}-\dfrac{a}{\sqrt{3}}\sqrt{\dfrac{3}{2a}}-=\dfrac{47}{10}\sqrt{2a}\)
⇒ Chọn C
