One of the following values for $a$ makes the expression $\dfrac{2a+5}{a^2+1}$ undefined. Which one?

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The expression is undefined when the denominator is equal to 0.

$$a^2+1 \neq 0$$ $$a^2 \neq -1$$ $$a \neq \pm\sqrt{-1}$$ $$a \neq \pm i$$