Square of i
From specialfunctionswiki
Theorem
The following formula holds: $$i^2=-1,$$ where $i$ denotes the imaginary number.
Proof
From the definition of $i$, $$i=\sqrt{-1}.$$ Squaring both sides shows $$i^2 = \left( \sqrt{-1} \right)^2 = -1,$$ as was to be shown.