Why this is wrong?

MatheusMkalo

(-3)² = ($$\sqrt{9}$$)² --> True

-3 = $$\sqrt{9}$$ --> False

Wolfram Alpha result: False

Last edited:

chiro

Because -3 doesn't equal 3?

micromass

We had a thread about this recently. The point is that $$\sqrt{x^2}\neq x$$. The square root of x is defined as the unique POSITIVE number who's square is x. Thus $$\sqrt{(-3)^2}$$ is the unique positive number who's square is $$(-3)^2=9$$, obviously, this number is 3.

Thus, in general, we have that $$\sqrt{x^2}=|x|$$.

$$(-3)^2=(\sqrt{9})^2$$

is true, since $$(-3)^2=9$$and $$\sqrt{9}=3$$ and $$3^2=9$$

But when taking the square root of both sides we get

$$\sqrt{(-3)^2}=\sqrt{(\sqrt{9})^2}$$

which evaluates to $$3=\sqrt{9}$$, which is perfectly true!!

chiro

Building on what micromass just said, use the horizontal line test to find out that there is no unique inverse except for the case when x = 0.

KingNothing

When you "undo" a squared operation, it is not as simple as just removing the little "2" superscript.

The square root of x is defined as the unique POSITIVE number who's square is x.
Really? In my classes, we've always used $$\sqrt[]{x^{2}}=\pm x$$

Mentallic

Homework Helper
When you "undo" a squared operation, it is not as simple as just removing the little "2" superscript.

Really? In my classes, we've always used $$\sqrt[]{x^{2}}=\pm x$$
No, this is wrong. The square root function is defined to be the unique positive square root of the number. $$\sqrt{9}=3$$ and if $$\sqrt{9}=\pm3$$ then I have no clue why we would write such answers as $$x^2=9 \rightarrow x=\pm \sqrt{9}$$ when the square root is already $$\pm$$ Char. Limit

Gold Member
And this is why when you use a multi-valued operator to prove something, you always have to check and see if the answer fits the original equation. Or you'll end up with extraneous or wrong solutions like -3=3.

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