ĐK: $a>0, a\ne 1$
$\Big( \dfrac{ \sqrt{a} }{2}-\dfrac{1}{2\sqrt{a}} \Big)^2.\Big( \dfrac{\sqrt{a}-1}{\sqrt{a}+1}-\dfrac{\sqrt{a}+1}{\sqrt{a}-1}\Big)$
$=\Big(\dfrac{ a-1}{2\sqrt{a}}\Big)^2.\Big( \dfrac{ (\sqrt{a}-1)^2-(\sqrt{a}+1)^2 }{a-1}\Big)$
$=\dfrac{(a-1)^2}{4a}.\dfrac{ a-2\sqrt{a}+1-a-2\sqrt{a}-1 }{(a-1)}$
$=\dfrac{a-1}{4a}.(-4\sqrt{a})$
$=-\dfrac{a-1}{\sqrt{a}}$
