Jun 13, 2020 · Is there a formal proof for $(-1) \\times (-1) = 1$?
It's a fundamental formula not only in arithmetic but also in the whole of math.
Is there a proof for it or is it just assumed?
Aug 24, 2016 · 1 Indeed what you are proving is that in the complex numbers you don't have (in general) $$\sqrt {xy}=\sqrt {x}\sqrt {y}$$ Because you find a counterexample.
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