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Is integer x<−20?

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Manager
Joined: 06 Jun 2014
Posts: 86
Location: United States
Concentration: Finance, General Management
GMAT 1: 450 Q27 V21
GPA: 3.47

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06 Feb 2016, 18:48
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74% (02:04) correct 26% (01:40) wrong based on 181 sessions

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Is integer x<−20?

1) $$x^2+40x+391=0$$

2) $$x^2= 529$$
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Joined: 06 Nov 2014
Posts: 1877

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06 Feb 2016, 22:17
1
zxcvbnmas wrote:
Is integer x<−20?

1) $$x^2+40x+391=0$$

2) $$x^2= 529$$

Statement 1: $$x^2+40x+391=0$$
$$x^2+23x +17x+391=0$$
(x+23)(x+17) = 0

Therefore x = -23 or x = -17
Insufficient

Statement 2: $$x^2= 529$$
x = -23 or 23
Insufficeint

From Statement 1 and 2, we know that x = -23
Sufficient. Option C
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Joined: 30 May 2017
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29 Jun 2017, 15:51
Are there any tips for finding the square root of large numbers like this?
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Joined: 24 Apr 2016
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29 Jun 2017, 16:08
Statement 2: $$x^2=529$$

x can be -23 or +23

As this statement gives two different values, This statement is not sufficient

Statement 1:$$x^2+40x+391=$$0

Above can be written as (x+23)(x+17)=0

So x can be -23 or -17

As this statement gives two different values, This statement is not sufficient

Combining both statements we get x=-23

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Joined: 24 Apr 2016
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29 Jun 2017, 16:15
1
Smokeybear00 wrote:
Are there any tips for finding the square root of large numbers like this?

Smokeybear00

To know what the square root of 529, you can say start with squaring a number which can be done easily in your head and gets u closer to the correct number

Lets straight go to 20. Square of 20 gives 400. So we are near by,
Let see 25. Square of 25 gives 625. So we know the number is between 20 and 25.

Now we know see the last digit of 529 is 9, so the unit's digit of the number we are looking for should be with 3 or 9. Between 20 and 25 only 23 satisfies this. So 23 is the number we are looking for.

This is how I try to do. I am sure others might have a better way.

The other thing i suggest is memorize the squares from 1-50. This will help a long way.
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23 Aug 2018, 11:54
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Re: Is integer x<−20?   [#permalink] 23 Aug 2018, 11:54
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