Simplifying $\Big[\dfrac{5-\sqrt{a}}{5+\sqrt a}-\dfrac{\sqrt a+5}{\sqrt a-5}+2\Big]^{-2}$ Simplifying $$\Big[\dfrac{5-\sqrt{a}}{5+\sqrt a}-\dfrac{\sqrt a+5}{\sqrt a-5}+2\Big]^{-2}$$ 
When I try, the numerator cancels out to $0$, yet the answer sheet says $(25-a)^2/10000$. Where am I going wrong & how is getting $10 000$ there even possible? 

Let $s = \sqrt{a}$:
$$
\frac{5-s}{5+s}-\frac{s+5}{s-5}
= \frac{-(s-5)^2 - (s+5)^2}
       {(5+s)(5-s)}
= \frac{(s-5)^2 + (s+5)^2}
       {s^2-5^2}
= \frac{2s^2 + 2 \cdot 5^2}{s^2-5^2}
= \frac{2(a + 25)}{a-25}
$$
Can you finish the problem?
