7. If $x=7+\sqrt{40}$, find the value of $\sqrt{x}+\frac{1}{\sqrt{x}}$
8. Prove that: $\frac{a^{-1}}{a^{-1}+b^{-1}}+\frac{a^{-1}}{a^{-1}-b^{-1}}=\frac{2b^2}{b^2-a^2}$