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If ab > 0, is ab^2 > 0 ?

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Math Revolution GMAT Instructor
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If ab > 0, is ab^2 > 0 ?  [#permalink]

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New post 16 Oct 2017, 01:55
1
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A
B
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D
E

Difficulty:

  45% (medium)

Question Stats:

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[GMAT math practice question]

If ab > 0, is ab^2 > 0 ?

1) a > 0
2) b > 0

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Re: If ab > 0, is ab^2 > 0 ?  [#permalink]

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New post 16 Oct 2017, 02:32
From Q stem we can conclude both a and b can be positive or both can be negative.

From st1 we can conclude a is positive. Hence b is also positive. So the square of the product will be positive

St2 also says same as above. Hence the square of the product will be positive.
Hence D

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Re: If ab > 0, is ab^2 > 0 ?  [#permalink]

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New post 16 Oct 2017, 03:17
Given ab > 0
So a and b both should be same sign.

1. a>0 so b should be greater than 0 so ab^2 should be grater than 0. Sufficient
2. b>0 so a should be greater than 0 so ab^2 should be grater than 0. Sufficient.

So D should be answer
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Re: If ab > 0, is ab^2 > 0 ?  [#permalink]

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New post 18 Oct 2017, 03:13
=>

Forget conventional ways of solving math questions. In DS, VA (Variable Approach) method is the easiest and quickest way to find the answer without actually solving the problem. Remember that equal number of variables and independent equations ensures a solution.

The first step of VA(Variable Approach) method is modifying the original condition and the question, and rechecking the number of variables and the number of equations.

Under the condition ab > 0, the question ab2 > 0 is equivalent to a > 0.

The condition 1) is equivalent to the question a > 0. This is sufficient.
And under the original condition ab > 0, the condition 2) b > 0 is equivalent to a > 0. This is sufficient too.

Therefore, the answer is D.

By the tip 1) of VA method, since two conditions are same, we can choose D as the answer.

Ans: D
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Re: If ab > 0, is ab^2 > 0 ? &nbs [#permalink] 18 Oct 2017, 03:13
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