Using DeMoivre's theorem, and the fact that any root in the first quadrant has argument between [tex]0^\circ[/tex] and [tex]90^\circ[/tex], we get
[tex]16i=16\left(\cos90^\circ+i\sin90^\circ)\implies\sqrt{16i}=\sqrt{16}\left(\cos45^\circ+i\sin45^\circ\right)[/tex]
so the missing value is 45.
