`B=(\frac{2}{3+\sqrt{5}}+\frac{1}{2+\sqrt{5}}).\sqrt{6+2\sqrt{5}}`
`=(\frac{2(3-\sqrt{5})}{9-5}+\frac{\sqrt{5}-2}{5-4}).\sqrt{(\sqrt{5}+1)^2}`
`=(\frac{3-\sqrt{5}}{2}+\frac{2\sqrt{5}-4}{2})(\sqrt{5}+1)`
`=(\frac{-1+\sqrt{5}}{2})(\sqrt{5}+1)`
`=\frac{(\sqrt{5}-1)(\sqrt{5}+1)}{2}`
`=\frac{5-1}{2}`
`=4/2=2`
`2)\sqrt{4x-20}+\sqrt{x-5}-1/3\sqrt{9x-45}=4(x>=5)`
`<=>2\sqrt{x-5}+\sqrt{x-5}-\sqrt{x-5}=4`
`<=>2\sqrt{x-5}=4`
`<=>\sqrt{x-5}=2`
`<=>x-5=4`
`<=>x=9(tm)`
Vậy `S={9}`
