a) ĐKXĐ: \(a>1;a\ne-1\) 

\(B=\left(\dfrac{3}{\sqrt{1+a}}+\dfrac{\sqrt{1-a}.\sqrt{1+a}}{\sqrt{1+a}}\right):\dfrac{3+\sqrt{1-a^2}}{\sqrt{1-a^2}}\)

\(\Leftrightarrow B=\dfrac{3+\sqrt{1-a}.\sqrt{1+a}}{\sqrt{1+a}}.\dfrac{\sqrt{1+a}.\sqrt{1-a}}{3+\sqrt{1+a}.\sqrt{1-a}}\)

\(\Leftrightarrow B=\sqrt{1-a}\)

b) Thay a=\(\dfrac{\sqrt{3}}{2+\sqrt{3}}\) vào B ta được:

\(B=\sqrt{1-\dfrac{\sqrt{3}}{2+\sqrt{3}}}\) 

\(\Leftrightarrow B\) \(=\sqrt{\dfrac{2+\sqrt{3}-\sqrt{3}}{2+\sqrt{3}}}\)

\(\Leftrightarrow B\) \(=\sqrt{\dfrac{2}{2+\sqrt{3}}}\) 

\(\Leftrightarrow B\)\(=\sqrt{\dfrac{4}{4+2\sqrt{3}}}\) \(\Leftrightarrow B\) \(=\dfrac{\sqrt{4}}{\sqrt{3+2\sqrt{3}+1}}\) 

\(\Leftrightarrow B=\dfrac{2}{\sqrt{\left(\sqrt{3}+1\right)^2}}\) \(\Leftrightarrow B=\dfrac{2}{\sqrt{3}+1}=\dfrac{2.\left(\sqrt{3}-1\right)}{3-1}=\sqrt{3}-1\) 

c) Có \(\sqrt{B}>B\) \(\Leftrightarrow\sqrt{\sqrt{1-a}}>\sqrt{1-a}\) 

\(\Leftrightarrow\sqrt{1-a}>1-a\) 

\(\Leftrightarrow\sqrt{1-a}-\left(1-a\right)>0\) 

\(\Leftrightarrow\sqrt{1-a}.\left(1-\sqrt{1-a}\right)>0\)

\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}\sqrt{1-a}>0\\1-\sqrt{1-a}>0\end{matrix}\right.\\\left\{{}\begin{matrix}\sqrt{1-a}< 0\\1-\sqrt{1-a}< 0\end{matrix}\right.\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}a< 1\\a>0\end{matrix}\right.\\\left\{{}\begin{matrix}a>1\\a< 0\end{matrix}\right.\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}0< a< 1\\a>1;a< 0\end{matrix}\right.\)