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Difficulties with Absolute Value

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Difficulties with Absolute Value

by garuhape » Wed Mar 02, 2011 4:00 am
I thought that I do not have any probs with absolute value questions, but apparently I do.

That's the prob with solution. IMO the correct answer is "all three are correct".

I do not understand why III is incorrect. Why do they say in the second last line that x = +- |x| = -x . Doesn't the +- mean that we can choose between + and - since the questions say which of the following could be correct.

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by sanju09 » Wed Mar 02, 2011 5:21 am
garuhape wrote:I thought that I do not have any probs with absolute value questions, but apparently I do.

That's the prob with solution. IMO the correct answer is "all three are correct".

I do not understand why III is incorrect. Why do they say in the second last line that x = +- |x| = -x . Doesn't the +- mean that we can choose between + and - since the questions say which of the following could be correct.

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by fskilnik@GMATH » Wed Mar 02, 2011 7:06 am
Hi there!

You are 100% correct, garuhape!

The explanation is bad-written, and the question stem is bad-posed because "+ or - something" is bad notation. (I will explain why at the end of this post.)

[In the problem posed, the usual interpretation of the notation will give the answer "all of them", as garuhape believes and correctly explores at the end of his post. What I mean is that I do not like the "+ or -" notation ANYWHERE.]

Just to rephrase what you said: when x is positive, then |x| = x therefore x IS equal to |x| or -|x|, because in this case (x>0) we have the first choice: "is x equal to x or to -x ?" Yes, x is equal to x...

Regards,
Fabio.

Explanation promised: I like to write that the set of integers is given by {0, 1, -1, 2, -2, 3, -3, ...} and I do NOT like to write that the set of integers is given by {0, +- 1, +- 2, ...} why? because I believe the student "has the right" to understand (wrongly) that I am "giving the option of choosing between say 1 and -1", therefore it seems as 1 or -1 could be left out of the set of integers, what is NOT the case, for sure. I hope you got the point!

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