Wilson:  [The solution is that

Is always true?  We proofed that in a way the square of , the square of and the square of .  So we concluded that .  But the truth is x²=y and , rather than only positive, .  In any case, we have no reason to suspicious whether it is positive or negative, because we don't deal with complex number in the real life.  And the trap is set for people who make this usual (but not necessary true) assumption.

In fact, , which will settle the solution.]