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Manager  Joined: 08 May 2015
Posts: 87
GMAT 1: 630 Q39 V38 GMAT 2: 670 Q44 V38 GMAT 3: 750 Q49 V44 If r, a and b are positive integers; is r a product of two consecutive  [#permalink]

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Difficulty:   45% (medium)

Question Stats: 70% (01:53) correct 30% (01:35) wrong based on 69 sessions

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If r, a and b are positive integers, is r a product of two consecutive odd integers?

1) $$r = 4a^2 - b^2$$
2) b = 1
CEO  S
Joined: 20 Mar 2014
Posts: 2560
Concentration: Finance, Strategy
Schools: Kellogg '18 (M)
GMAT 1: 750 Q49 V44 GPA: 3.7
WE: Engineering (Aerospace and Defense)
Re: If r, a and b are positive integers; is r a product of two consecutive  [#permalink]

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2
Mascarfi wrote:
If r, a and b are positive integers, is r a product of two consecutive odd integers?

1) $$r = 4a^2 - b^2$$
2) b = 1

2 Consecutive odd integers can be of the form : [2a+1 , 2a-1] or [2a+1, 2a+3] , with a $$\in$$ integer.

Now, statement 2 is clearly not sufficient.

Per statement 1, $$r = 4a^2 - b^2$$ ---> r = (2a+b)(2a-b). Thus r can be equal to product of 2 consecutive odd integers only if b =1 .

But per this statement, b can be any value (=3,5,4, etc.). Thus r = (2a+4)(2a-4) (this would give r = even and thus given question will be "no") or r = (2a+1)(2a-1) (this will give r = odd and thus the given question will be "yes").

Thus statement 1 is not sufficient.

Combining statements 1 and 2, we see r = (2a+1)(2a-1) and thus r is a product of 2 consecutive odd integers. Hence we get a definite answer by combining the statements and hence C is the correct answer.
Senior Manager  B
Joined: 28 Jun 2015
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Concentration: Finance
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Re: If r, a and b are positive integers; is r a product of two consecutive  [#permalink]

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2
Let the odd numbers be $$(2x-1)$$ and $$(2x+1)$$.

$$r$$ must be of the form $$(2x-1) (2x+1) = 4x^2 - 1$$.

(i) $$r = 4a^2 - b^2$$: In order for $$r$$ to be of the form $$(4x^2 - 1)$$, $$b^2$$ must be an odd number, but we have no idea what b is, so not sufficient.

(ii) $$b = 1$$. clearly not sufficient.

(i) + (ii): clearly sufficient. Ans (C).
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Re: If r, a and b are positive integers; is r a product of two consecutive  [#permalink]

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_________________ Re: If r, a and b are positive integers; is r a product of two consecutive   [#permalink] 21 Aug 2017, 14:16
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