usre123
I'm a bit unclear about this. Doesn't this simply mean x=x? I really didn't know what to do with this statement...
if we had -4, then\(\sqrt{(-4)^2}\) would still give a -4, wouldn't it?
You
really need to brush up fundamentals on roots and absolute values. This is basic staff!
First, of all,
when the GMAT provides the square root sign for an even root, such as a square root, fourth root, etc. then the only accepted answer is the positive root. So, \(\sqrt{(-4)^2}=\sqrt{16}=4\), not -4 and not +/-4, ONLY 4!
Next, about \(\sqrt{x^2}=|x|\).
The point here is that as
square root function can not give negative result then \(\sqrt{some \ expression}\geq{0}\).
So \(\sqrt{x^2}\geq{0}\). But what does \(\sqrt{x^2}\) equal to?
Let's consider following examples:
If \(x=5\) --> \(\sqrt{x^2}=\sqrt{25}=5=x=positive\);
If \(x=-5\) --> \(\sqrt{x^2}=\sqrt{25}=5=-x=positive\).
So we got that:
\(\sqrt{x^2}=x\), if \(x\geq{0}\);
\(\sqrt{x^2}=-x\), if \(x<0\).
What function does exactly the same thing? The absolute value function: \(|x|=x\), if \(x\geq{0}\) and \(|x|=-x\), if \(x<0\). That is why \(\sqrt{x^2}=|x|\).
Best GMAT Math Prep Books (Reviews & Recommendations):
best-gmat-math-prep-books-reviews-recommendations-77291.htmlHope it helps.
thank you for taking the time out to explain... I'm studying math after a very very long time so I'm extremely rusty and most of my questions involve very very basic concepts.
I just reversed the OG13 DS question one concept that when x^2=4, x can be + or-2.
then perhaps underoot 4 can be + or -ve 2 as well.
I read this post, and perhaps this mixed me up (or I suppose I fail to understand rule 3). Could someone please comment?