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\(B=\left(\frac{x}{2\sqrt{x}}-\frac{1}{2\sqrt{x}}\right)^2.\left(\frac{\left(\sqrt{x}-1\right)^2-\left(\sqrt{x}+1\right)^2}{\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)}\right)\)
\(=\left(\frac{x-1}{2\sqrt{x}}\right)^2\left(\frac{x-2\sqrt{x}+1-x-2\sqrt{x}-1}{x-1}\right)\)
\(=\frac{\left(x-1\right)^2}{4x}.\frac{-4\sqrt{x}}{\left(x-1\right)}=\frac{1-x}{\sqrt{x}}\)
\(B=-2\Leftrightarrow\frac{1-x}{\sqrt{x}}=-2\Leftrightarrow1-x=-2\sqrt{x}\)
\(\Leftrightarrow x-2\sqrt{x}-1=0\Rightarrow\left[{}\begin{matrix}\sqrt{x}=1+\sqrt{2}\\\sqrt{x}=1-\sqrt{2}< 0\left(l\right)\end{matrix}\right.\)
\(\Rightarrow x=\left(1+\sqrt{2}\right)^2=3+2\sqrt{2}\)
