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I don't understand why it also can be negative

Why is $x^2 = f(a) => x = \sqrt{f(a)}$ AND $-\sqrt{f(a)}$

nonuser
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ScoobyDuh
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  • $x=\pm y$ is an abbreviation for "$x=y$ OR $x=-y$" And in math, "A or B" does NOT mean that one of A,B is true and the other false. If $x=\pm y$ and $y=0$ then $x=y$ and $x=-y$ are both true. – DanielWainfleet Nov 08 '18 at 19:45

1 Answers1

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Actualy it is OR and not AND. $x$ can be one of them, it can't be both at the same time. It is equivalent to $$x^2-\sqrt{f(a)}^2=0$$ so

$$(x-\sqrt{f(a)})(x+\sqrt{f(a)})=0$$

nonuser
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