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15 Sep 2014, 23:45



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23 Oct 2014, 23:32
Bunuel  Could you please look into why (2) alone is not sufficient ? We have ac > 0. \(a^7*b^2*c^3 = a^4 * b^2 * a^3 * c^3 = a^4 * b^2 * (ac)^3\). All 3 are positive, so the whole expression is positive. B should be the answer I feel.



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24 Oct 2014, 01:08
anubhavmax wrote: Bunuel  Could you please look into why (2) alone is not sufficient ? We have ac > 0. \(a^7*b^2*c^3 = a^4 * b^2 * a^3 * c^3 = a^4 * b^2 * (ac)^3\). All 3 are positive, so the whole expression is positive. B should be the answer I feel. Hi anubhav... How about of b=0 then the expression a^7*b^2*c*3=0 So you need information that terms are not zero..B tells you a and c have same sign but not whether b =0 or not hope it helps
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24 Oct 2014, 01:42
Oh right... thanks a lot. Simply overlooked the zero case.



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10 Dec 2014, 19:20
bc<0 ac>0
here we have: either b is negative, then a and c are positive, in this case a^7*b^2*c*3<0 or b is positive, and a and c are negative, in this case a^7*b^2*c*3>0 from where did we get b not 0?



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23 Jun 2016, 02:30
I think this is a highquality question and I agree with explanation. I need one point clarification from Bunuel is:
when you say bc < 0 it is possible that c as well not equal to zero along with b. So we have some information on C also right. May be I am not getting your point.



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20 Aug 2016, 03:48
I think this is a highquality question and I agree with explanation.



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16 Jan 2017, 10:37
I think this is a highquality question and I agree with explanation.



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Bunuel wrote: Is \(a^7*b^2*c^3 \gt 0\)?
(1) \(bc \lt 0\)
(2) \(ac \gt 0\) Hi Bunuel If a^7 = that can indicate A can be (+\) going ahead , * B2 = this means B will be every time positive, C^3 , C may be +\ values coming back to the main equation , if we multiply Negative * positive * Negative = this gives us a positive value . Is my mistake only that I haven't considered Zero in my values ? As by even / odd concept , I had marked this D .
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05 Jun 2018, 03:39
loserunderachiever wrote: Bunuel wrote: Is \(a^7*b^2*c^3 \gt 0\)?
(1) \(bc \lt 0\)
(2) \(ac \gt 0\) Hi Bunuel If a^7 = that can indicate A can be (+\) going ahead , * B2 = this means B will be every time positive, C^3 , C may be +\ values coming back to the main equation , if we multiply Negative * positive * Negative = this gives us a positive value . Is my mistake only that I haven't considered Zero in my values ? As by even / odd concept , I had marked this D . x^odd can be positive, negative or 0. x^even can only be positive, or 0. Not sure I understand what's your question though.
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05 Jun 2018, 03:41
Bunuel wrote: loserunderachiever wrote: Bunuel wrote: Is \(a^7*b^2*c^3 \gt 0\)?
(1) \(bc \lt 0\)
(2) \(ac \gt 0\) Hi Bunuel If a^7 = that can indicate A can be (+\) going ahead , * B2 = this means B will be every time positive, C^3 , C may be +\ values coming back to the main equation , if we multiply Negative * positive * Negative = this gives us a positive value . Is my mistake only that I haven't considered Zero in my values ? As by even / odd concept , I had marked this D . x^odd can be positive, negative or 0. x^even can only be positive, or 0. Not sure I understand what's your question though. I got your point , as I didn't consider X to be Zero . Thanks.
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29 Sep 2018, 01:03
Bunuel wrote: Official Solution:
Inequality \(a^7*b^2*c^3 \gt 0\) to be true \(a\) and \(c\) must be either both positive or both negative AND \(b\) must not be zero (in order \(a^7*b^2*c^3\) not to equal zero). (1) \(bc \lt 0\). Hence, \(b \ne 0\). Don't know about \(a\) and \(c\). Not sufficient. (2) \(ac \gt 0\). Hence, \(a\) and \(c\) are either both positive or both negative. Don't know about \(b\): if \(b=0\), then the expression will be equal 0. Not sufficient. (1)+(2) Sufficient.
Answer: C Hi, How can we say 1 + 2 is sufficient?(What if a=0 or c=0) Because we have not considered a=0 or c=0 case. Why have we not considered this and only considered b=0 case? Can you please explain?



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29 Sep 2018, 06:08
Akshit03 wrote: Bunuel wrote: Official Solution:
Inequality \(a^7*b^2*c^3 \gt 0\) to be true \(a\) and \(c\) must be either both positive or both negative AND \(b\) must not be zero (in order \(a^7*b^2*c^3\) not to equal zero). (1) \(bc \lt 0\). Hence, \(b \ne 0\). Don't know about \(a\) and \(c\). Not sufficient. (2) \(ac \gt 0\). Hence, \(a\) and \(c\) are either both positive or both negative. Don't know about \(b\): if \(b=0\), then the expression will be equal 0. Not sufficient. (1)+(2) Sufficient.
Answer: C Hi, How can we say 1 + 2 is sufficient?(What if a=0 or c=0) Because we have not considered a=0 or c=0 case. Why have we not considered this and only considered b=0 case? Can you please explain? How can either of them be 0 if it's given that ac > 0?
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