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alex.gellatly
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is svx<0?
Source: Beat The GMAT — Data Sufficiency |
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Brent@GMATPrepNow
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deleted
Last edited by Brent@GMATPrepNow on Wed Feb 25, 2015 7:36 am, edited 2 times in total.
Brent Hanneson - Creator of GMATPrepNow.com
Brent you have made a mistake. The correct answer should be E. Let me explain:Brent@GMATPrepNow wrote:alex.gellatly wrote:If s^4v^3x^7<0, is svx<0?
1. v<0
2 x>0
thanks
Two important rules:
Odd exponents preserve the sign of the base.
That is, a positive number raised to an odd power will remain positive. A negative number raised to an odd power will remain negative.
An even exponent always yields a positive result. (Proviso: As long as the base does not equal zero.)
In other words, any non-zero number raised to an even power will result in a positive value.
Since we're told that s^4v^3x^7 < 0, we can conclude that s, v and x do not equal zero. Furthermore, we can conclude that s^4 is positive (from the rule above).
So, we get (some positive number)(v^3)(x^7) < 0
We also know that v^3 has the same sign (positive or negative) as v, and we know that x^7 has the same sign as x (from the rule above).
Since (some positive number)(v^3)(x^7) is negative, there are only two possible cases:
case a) v is positive and x is negative
case b) v is negative and x is positive
Target question: Is svx<0?
Statement 1: v is negative
This means that case b is true, and we can rule out case a.
If case b is true, then v is negative and x is positive, which means (s^4)(v^3)(x^7) = (positive)(negative)(positive) = some negative number.
In other words, (s^4)(v^3)(x^7) must be less than 0
SUFFICIENT
Statement 2: x is positive
This means that case b is true, and we can rule out case a.
Same logic as before . . . (s^4)(v^3)(x^7) must be less than 0
SUFFICIENT
Answer = D
Cheers,
Brent
We are given s^4*v^3*x^7<0 which tells us two things.
1. None of s, v, x are 0.
2. s^4*v^3*x^7 = (s^2*v*x^3)^2*(vx) <0
anything squared is positive (since none of s,v,x equals 0), hence vx<0.
We are looking for svx. We know that vx<0, so we need to find a statement that tells us whether s is greater than or less than 0.
Now we have the two conditions. :
1. v<0 , doesn't tell whether s is positive or negative, INSUFFICIENT.
2 x>0, doesn't tell whether s is positive or negative, INSUFFICIENT.
Together, it tells us that v<0, x>0 tell us that vx<0, which we already knew. Hence we still don't know the sign of s. Still INSUFFICIENT. Hence E
Let me know if this helps
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Brent@GMATPrepNow
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Deleted for the sake of the children.
Cheers,
Brent
Cheers,
Brent
Last edited by Brent@GMATPrepNow on Tue Jun 05, 2012 6:23 pm, edited 2 times in total.
Brent Hanneson - Creator of GMATPrepNow.com
Even I am getting the answer as E.
since we do not know about sign of s.
Please explain...
since we do not know about sign of s.
Please explain...
If my post helped you- let me know by pushing the thanks button. Thanks
GMAT/MBA Expert
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Brent@GMATPrepNow
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Arrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrggh, WOW!!!1947 wrote:Even I am getting the answer as E.
since we do not know about sign of s.
Please explain...
This whole time I've been answering the wrong target question!
I've been answering the target question: "Is (s^4)(v^3)(x^7) < 0?"
So, in both statements, I grandly concluded that "(s^4)(v^3)(x^7) must be less than 0"
Of course, the REAL target question is: "Is svx<0?"
So, please disregard all of my posts in this thread. Since we never find out the sign of s, the answer is E.
Aside: Interestingly enough, the title of my next (upcoming) article for BTG happens to be "How to Avoid Making Silly Mistakes." I guess I'll have read that article
Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
looking forward to your new article...i have been doing these silly mistakes specially in DS.
I usually miss
- inequality condition
- if a variable is already an integer
so after all the work...i make a silly mistake
I usually miss
- inequality condition
- if a variable is already an integer
so after all the work...i make a silly mistake
If my post helped you- let me know by pushing the thanks button. Thanks
