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? 

Note that the expression in the brackets can be adapted by changing the sign of the second term and the signs in the denominator. Simplify to  $$\frac {5-\sqrt a}{5+\sqrt a}+\frac {5+\sqrt a}{5-\sqrt a}+2=\frac {(5-\sqrt a)^2+(5+\sqrt a)^2+2(25-a)}{25-a}=\frac {100}{25-a}$$
