2
$\begingroup$

This question already has an answer here:

For example, $\sqrt{4}$. I've asked a bunch of people and I get mixed answers all the time, as to whether it is $-2$ and $+2$ or just $+2$.

How about if there's a negative in front of the square root sign, for example, $-\sqrt{4}$? Would that still be plus or minus or just minus?

$\endgroup$

marked as duplicate by Rahul, choco_addicted, Strants, Claude Leibovici, Watson May 10 '16 at 7:07

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.

  • 2
    $\begingroup$ Square root is always positive. It is defined to be positive. $\endgroup$ – Apurv May 10 '16 at 2:20
  • 1
    $\begingroup$ So the square root of 4 is just positive two? $\endgroup$ – John May 10 '16 at 2:22
  • 3
    $\begingroup$ Note that $\sqrt {x^2}=|x|$, which is always positive. You cannot simply cancel the exponent. You have to make sure that you get a positive number. The graph of the square root function ($y=\sqrt x$) is defined to stay in the first quadrant. $\endgroup$ – Apurv May 10 '16 at 2:24
  • 1
    $\begingroup$ $\sqrt {(-5)^2}=|-5|=5$, as per the definition I just gave in the previous comment. $\endgroup$ – Apurv May 10 '16 at 2:43
  • 1
    $\begingroup$ First Question: It is a matter of mathematical convention that $\sqrt{x}\ge0$ for all $x\ge0$, so that $\sqrt{x}$ is well-defined and has a single value. Second Question: $(-5)^2=25$ hence $\sqrt{(-5)^2}=\sqrt{25}=5$.$. $\endgroup$ – Mirko May 10 '16 at 2:53
3
$\begingroup$

This is a common source of confusion, because people don't clearly separate in their minds a few related but different situations.

  • Does $\sqrt4$ mean $\pm2$ or just $2$? The answer: it means $2$. By definition, the $\sqrt{\cdot}$ function always evaluates to a nonnegative number (as long as it's being applied to a nonnegative number; otherwise it's not defined at all). The definition of $\sqrt x$ is: the nonnegative number $y$ such that $y^2=x$.
  • Does $(\sqrt x)^2$ always equal $x$? (We'd better assume that $x$ itself is nonnegative for the notation to make sense.) The answer: yes. This is exactly the definition of the $\sqrt\cdot$ function, as described above.
  • Does $\sqrt{x^2}$ always equal $x$? The answer: no, since "the nonnegative number whose square is $x^2$" is not always $x$. But $|x|$ is a nonnegative number and its square is $x^2$; therefore $\sqrt{x^2}=|x|$.
  • Suppose that $x^2=4$; does that mean that $x=\pm2$ or just $x=2$? The answer: it means $x=\pm2$. If we apply the square root function to both sides of the equation $x^2=4$, we get $|x|=2$, which is equivalent (by the definition of absolute value) to $x=\pm2$.
$\endgroup$

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