Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

This question already has an answer here:

Are they equal?

-5 = $\sqrt{(-5)^2}$

share|improve this question

marked as duplicate by J. M., Andreas Caranti, muzzlator, TMM, Ron Gordon Apr 5 '13 at 9:09

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
No, it's --5.... –  Kaster Apr 5 '13 at 8:04

4 Answers 4

up vote 0 down vote accepted

The motivation for the definition of the function $\sqrt{x}$ is simply as the inverse of the function $x^2$.

But the function $f(x)=x^2$ is even and hence is not one one. And hence the inverse function should return you two values. But just to make the inverse function actually a function(a relation which have all members in the domain having exactly one image) we define $\sqrt{x}$ as a function which gives you the positive root .

So most generally the inverse of the function $f(x)=x^2$ (let's call the inverse as $g(x)$ is defined this way $$g(x)= {\begin{cases}\sqrt{x}\quad \\ -\sqrt{x} \end{cases}} $$


And the answer to the original question, as I have mentioned before $\sqrt{x}$ gives you only positive values and hence $\sqrt{(-5)^2}=5$ .

share|improve this answer

Square root always gives a positive result.

Hence the result for your question i.e.,$\sqrt{25}$ is $5$.

But if the question were, $a^2 = (-5)^2$ , then $a = +5$ (or) $-5$

share|improve this answer

As you are probably aware of, there are two solutions to the equation:

$$x^2 = 25$$

namely $x = \pm 5$. For $n > 0$, the radix $\sqrt{n}$ is defined to be the positive solution to $x^2 = n$. So in this case, $\sqrt{(-5)^2} = \sqrt{25} = 5$.

The other solution to $x^2 = 25$, i.e. $x = -5$, is written $-\sqrt n$ in general.

share|improve this answer
    
So do you mean this possessed 2 solutions? –  Roylee Apr 5 '13 at 8:13
    
There are two solutions to $x^2 = 25$. The number $\sqrt{25}$ is the positive solution for $x$. That is, $\sqrt{25} = 5$. To be explicit, $\sqrt{25} \ne -5$. –  Lord_Farin Apr 5 '13 at 8:16

There is a general rule : $\sqrt{x^2} = |x|$ for all real $x$.

So we have $\sqrt{(-5)^2} = |-5| = 5$.

share|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.