\textbf{107--} If $\dfrac{1}{a^2+1} + \dfrac{1}{a^2-1} = 2$, then $\left(\dfrac{1}{a^2 - \sqrt{a^2+1}} + \dfrac{1}{a^2 + \sqrt{a^2+1}}\right)^{150}$ equals what?
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\hspace{1cm} (1) $2$ \hspace{2cm} (2) $-2$ \hspace{2cm} (3) $1$ \hspace{2cm} (4) $-1$
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