Answer
Fill the blank with
$\pm\sqrt{d}$
Work Step by Step
When we write $\sqrt{d}$, we mean the primary, positive square root of d.
$(\sqrt{d})^{2}=d$ by the property of roots, but,
also,
$(-\sqrt{d})=(-1)^{2}(\sqrt{d})^{2}=d$
So, $u$ can be $+\sqrt{d}$ or $-\sqrt{d}$, which we write $\pm\sqrt{d}$.
The square root property:
If $u^{2}=d$ then $u=\pm\sqrt{d}$
Fill the blank with
$\pm\sqrt{d}$
