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1-0.000001=?

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Math Revolution GMAT Instructor
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1-0.000001=?  [#permalink]

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New post 19 Apr 2017, 01:38
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A
B
C
D
E

Difficulty:

  35% (medium)

Question Stats:

68% (01:07) correct 32% (01:06) wrong based on 253 sessions

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1-0.000001=?

A. 1.001*0.999
B. 1.0001*0.9999
C. 1.001*0.9999
D. 1.0001*0.999
E. 1.01*0.99

DISCUSSED HERE: https://gmatclub.com/forum/1-271956.html

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Re: 1-0.000001=?  [#permalink]

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New post 19 Apr 2017, 06:11
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Top Contributor
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MathRevolution wrote:
1 - 0.000001=?

A. 1.001 x 0.999
B. 1.0001 x 0.9999
C. 1.001 x 0.9999
D. 1.0001 x 0.999
E. 1.01 x 0.99


Notice that we have a difference of squares here: - = (a + b)(a - b)

1 = 1² and 0.000001 = 0.001²

So, 1 - 0.000001 = - 0.001²
= (1 + 0.001)(1 - 0.001)
= (1.001)(0.999)

Answer:

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Re: 1-0.000001=?  [#permalink]

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New post 19 Apr 2017, 07:06
3
I solved like this way,
1- 0.000001= 0.999999
since the answer has 6 decimal digits, the only possible choice is A :)
Math Revolution GMAT Instructor
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Joined: 16 Aug 2015
Posts: 7618
GMAT 1: 760 Q51 V42
GPA: 3.82
Re: 1-0.000001=?  [#permalink]

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New post 21 Apr 2017, 01:29
==> You get \(a^2-b^2=(a+b)(a-b)\), and thus \(1-0.000001=1^2-0.001^2=(1+0.001)(1-0.001)=1.001*0.999\).

The answer is A.
Answer: A
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Re: 1-0.000001=?  [#permalink]

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New post 19 Dec 2018, 21:46
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Re: 1-0.000001=?   [#permalink] 19 Dec 2018, 21:46
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