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If r, s, and t are nonzero integers, is r^5*s^3*t^4 negative

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If r, s, and t are nonzero integers, is r^5*s^3*t^4 negative [#permalink] New post 11 Apr 2012, 14:52
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If r, s, and t are nonzero integers, is r^5*s^3*t^4 negative?

(1) rt is negative
(2) s is negative

Could someone kindly render a more easily understood explanation than that found in the Quantitative Review 2nd Edition. It would be really appreciated.
[Reveal] Spoiler: OA
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Re: If r, s, and t are nonzero integers, is r^5*s^3*t^4 negative [#permalink] New post 11 Apr 2012, 15:11
Expert's post
dzodzo85 wrote:
If r, s, and t are nonzero integers, is r^5*s^3*t^4 negative?

(1) rt is negative
(2) s is negative

Could someone kindly render a more easily understood explanation than that found in the Quantitative Review 2nd Edition. It would be really appreciated.


Since r, s, and t are nonzero integers then in order r^5*s^3*t^4 to be negative, only one condition should hold: r and s must have the opposite signs, in this case (r^5*s^3)*t^4=(negative)*(positive)=negative. Notice that if we were not told that given variables are nonzero then there would be one more condition that t must not be zero.

(1) rt is negative --> r and t have the opposite signs. Not sufficient, since no info about s.
(2) s is negative. Clearly insufficient.

(1)+(2) If r is positive then the answer will be YES (since r^5*s^3*t^4=positive*negative*positive=negative) but if r is negative then the answer will be NO (r^5*s^3*t^4=negative*negative*positive=positive). Not sufficient.

Answer: E.

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Re: If r, s, and t are nonzero integers, is r^5*s^3*t^4 negative [#permalink] New post 13 Jun 2012, 10:56
Hi,

r^5s^3t^4 < 0?

Using (1);
rt is -ve,
so, r^5s^3t^4 = (rt)^4r^1s^3, no clue about s, Insufficient.

Using (2),
s is -ve, r & t are unknown, Insufficient.

Using (1) & (2),
(rt)^4r^1s^3
(rt)^4 > 0, and s < 0
so the expression reduces tor^1(-ve), but we still don't know if r is +ve/-ve.

Thus, Answer is (E).

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Re: If r, s, and t are nonzero integers, is r^5*s^3*t^4 negative [#permalink] New post 05 Sep 2012, 04:25
Once you realize that two exponents are odd and one is even, you know that the you only have to worry about the signs of r and s, thus reducing the complexity.
Picking numbers might also help, I think, even though it might take a bit too long.

Thank you Bunuel and Cyberjadugar.
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If r, s, and tare nonzero integers, is r5s3 t 4 negative? [#permalink] New post 28 Sep 2013, 06:52
If r, s, and tare nonzero integers, is r^5s^3t^4 negative?
(1) rt is negative.
(2) s is negative.
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Re: If r, s, and tare nonzero integers, is r5s3 t 4 negative? [#permalink] New post 28 Sep 2013, 06:58
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Re: If r, s, and tare nonzero integers, is r5s3 t 4 negative?   [#permalink] 28 Sep 2013, 06:58
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