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Help me win "The One Thing You Wish You Knew - GMAT Club Contest" http://gmatclub.com/forum/the-one-thing-you-wish-you-knew-gmat-club-contest-140358.html#p1130989

We know that if \((a-b)*c<0\) , either \(c<0\) or \((a-b)<0\) which means \(a<b\)

1. \(a<b\) true. 2. \(c<0\) true. 3. \(|c|<1\) Could be true since \(c\) could be \(-ve\) or \(+ve\). 4. \(ac>bc\) Now if \((a-b)\) is \(+ve\) then \(a>b\) but at the same time \(c\) is \(-ve\) so \(ac\) is less than \(bc\). On the other hand if \((a-b)\) is \(-ve\) than \(a<b\) and \(ac\) cannot be greater than \(bc\) since \(c\) is \(+ve\). 5. \(a^2-b^2>0\) Now from the original statement if \(c<0\) then \((a-b)>0\) so \(a>b\) and \(a^2\) and \(b^2\) are both \(+ve\) so this could be possible.

Hence Answer D
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"Nowadays, people know the price of everything, and the value of nothing."Oscar Wilde

If (a – b)c < 0, which of the following cannot be true?

A. a < b B. c < 0 C. |c| < 1 D. ac > bc E. a^2 – b^2 > 0

Time saving approach:

Given: \((a-b)c<0\).

Now, since only option D has all three unknowns in it, I'd analyze this answer choice first:

D. \(ac > bc\) --> \(ac-bc>0\) --> \((a-b)c>0\). As you can see this option says directly opposite of what is given in the stem, hence it must be false.

There is no need even to check other options, as you cannot have two correct answers.

Answer: D.

P.S. Having all three unknowns does not mean that D is automatically a correct answer. An option could have two or even one unknown and still be a correct answer. For example if one of the options were \(c=0\) or \(a-b=0\) (a=b) then it would mean that it's false, since in this case \((a-b)c=0\), which means that this option would have been a correct answer.

Re: If (a – b)c < 0, which of the following cannot be true? [#permalink]

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29 Jan 2014, 08:17

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Re: If (a – b)c < 0, which of the following cannot be true? [#permalink]

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13 Feb 2015, 23:35

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Since this question asks which of the following CANNOT be true, you will likely be able to disprove most (if not all) of the wrong answers with some simple examples. If we can prove that an answer IS possible (even just once), then we can eliminate it.

We're told that (A-B)(C) < 0

This means that one of the two parentheses MUST be POSITIVE while the other MUST be NEGATIVE. Recognizing THIS Number Property should speed you up - You can now either TEST VALUES or use this Number Property to eliminate answers.

Answer A: A < B

If A<B, then (A-B) is negative and C is positive. This IS possible. Eliminate A.

Answer B: C < 0

IF C<0, then (A-B) is positive. This IS possible. Eliminate B.

Answer C: |C| < 1

C can be positive or negative, so (A-B) would be the opposite. This IS possible. Eliminate C.

Between the remaining two answers, Answer E seems like the easier option to eliminate....

Answer E: A^2 - B^2 > 0

IF A>B and they're both positive, then (A-B) is positive and C is negative. This IS possible. Eliminate E.

Re: If (a – b)c < 0, which of the following cannot be true? [#permalink]

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12 Nov 2016, 07:05

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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