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Re: Inequalities and modules DS [#permalink]
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NOTE: a^3 + b^3 + c^3 = 3abc if a + b + c = 0
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Re: Inequalities and modules DS [#permalink]
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gmatpapa wrote:
If abc ≠ 0, what is the value of \(\frac{a^3 + b^3 + c^3}{abc}\) ?

(1) \(|a|=1, |b|=2, |c|=3\)
(2) \(a + b + c = 0\)


\(a^3+b^3+c^3=(a+b+c)(a^2+b^2+c^2-ab-bc-ac)+3abc\)

Using statement 2 & the above formula:
\(\frac{a^3 + b^3 + c^3}{abc} = \frac{(a+b+c)(a^2+b^2+c^2-ab-bc-ac)}{abc}+\frac{3abc}{abc} = 0+3=3\)
Sufficient.

Ans: "B"
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Re: Inequalities and modules DS [#permalink]
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By the way, are we expected to know these equations for GMAT? :o
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Re: Inequalities and modules DS [#permalink]
1. Not sufficient as there more than one possible combination for a, b and c that satisfies the given equation.

2. Sufficient
by solving the given equation we can see that
(a+b+c)^3 = a3+b3+c3-3abc

=> a3+b3+c3 = 3abc , enough to answer the question.

Answer is B.
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Re: Inequalities and modules DS [#permalink]
To add some more, this has been posted earlier, please take a look at the solution step suggesting c = -(a+b).

ds-1543.html
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Re: Inequalities and modules DS [#permalink]
I seriously don't know how to use this in real life. I am not good in algebra. Not my style :-(
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Re: If abc 0, what is the value of a^3 + b^3 + c^3/abc ? (1) [#permalink]
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gmatpapa wrote:
If abc ≠ 0, what is the value of \(\frac{a^3 + b^3 + c^3}{abc}\) ?

(1) \(|a|=1, |b|=2, |c|=3\)
(2) \(a + b + c = 0\)



Alternative approach

statement 1: Clearly there are many different values that will lead to various results . Insuff

statement 2: we plug numbers that stefiey the fact a+b+c=0

a=1, b= 1, c=-2........apply in question stem (1+1-8)/-2= -6/-2=3

a=1, b=2, c=-3.........apply in question stem (1+8-27)/-6=-18/-6=3

a=1, b=3, c=-4.........apply in question stem (1+27-64)/-12= -36/-12=3

clearly the answer is B
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Re: If abc 0, what is the value of a^3 + b^3 + c^3/abc ? (1) [#permalink]
AmrithS wrote:
By the way, are we expected to know these equations for GMAT? :o


I'm bumping this question. This would be difficult to complete in a timed environment. Experts, is this a realistic problem on the gmat?

Posted from my mobile device
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Re: If abc 0, what is the value of a^3 + b^3 + c^3/abc ? (1) [#permalink]
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(1) is clearly insufficient due to different possible values for the expression e.g. -1, -2, 3 or -1, 2, 3 etc.

(2) Try substitution method as follows:
a = -(b+c)
therefore, expression becomes a^3 = - (b^3 + 3b^2c + 3bc^2 + c3) ---> substitute into expression, you will get:
[-b^3+b^3 - c^3+c^3 -3(bc)(b+c)] / [-(b+c)(bc)]
= -3bc(b+c) / -(b+c)(bc)
= 3. (Sufficient)
No use of the identity formula if you do not recognize it. You would only need to know the formula (a+b)^3 = a^3 + 3a^2b + 3ab^2 + b^3.
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Re: If abc 0, what is the value of a^3 + b^3 + c^3/abc ? (1) [#permalink]
Hello from the GMAT Club BumpBot!

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Re: If abc 0, what is the value of a^3 + b^3 + c^3/abc ? (1) [#permalink]
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