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# Inequalities and Roots

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Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
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25 Jul 2013, 21:26
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gmatter0913 wrote:
Hi Karishma,

I tried the problem x-1 < sqrt (7-x) as below:

As 7-x is under sqrt, it is +ve. Therefore, 7-x>=0 ; x<=7 ----->(1)

x-1 can be -ve or +ve

When x-1<=0; x<=1 --------> (2)

When x-1>=0; x>=1 --------> (3)

As both sides are +ve, we can square both the sides

(x-1)^2 < 7-x
x^2 -x -6<0
(x-3)(x+2)<0

-2<x<3 ------------>(4)

The answer to this problem is (x<3). I am not sure how to arrive at that from hereon. Could you please help me?

You have done the process correctly. Now you need to understand what this implies.

You got x <= 7

Case 1: x-1< 0
When x-1< 0; x < 1
Note that when x-1 is negative, it will always be less than $$\sqrt{(7-x)}$$
So whenever x<1, the inequality will always hold.

Case 2: x-1 >= 0
If x-1 is non negative, we can square the inequality.
From this, you get -2<x<3.
The inequality holds in this range.

From the two cases, we see that the inequality holds for the range x < 3.
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Karishma
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Kudos [?]: 17357 [1], given: 232 Senior Manager Joined: 12 Mar 2010 Posts: 357 Kudos [?]: 270 [0], given: 87 Concentration: Marketing, Entrepreneurship GMAT 1: 680 Q49 V34 Re: Inequalities and Roots [#permalink] ### Show Tags 26 Jul 2013, 02:05 Hi Karishma, I have one more doubt on my solution posted earlier. Quote: I tried the problem x-1 < sqrt (7-x) as below: As 7-x is under sqrt, it is +ve. Therefore, 7-x>=0 ; x<=7 ----->(1) x-1 can be -ve or +ve When x-1<=0; x<=1 --------> (2) When x-1>=0; x>=1 --------> (3) As both sides are +ve, we can square both the sides (x-1)^2 < 7-x x^2 -x -6<0 (x-3)(x+2)<0 -2<x<3 ------------>(4) Shouldn't this be 1<=x<3 (as x>=1 is the pre-supposed condition to square them) Kudos [?]: 270 [0], given: 87 Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7674 Kudos [?]: 17357 [1], given: 232 Location: Pune, India Re: Inequalities and Roots [#permalink] ### Show Tags 26 Jul 2013, 04:23 1 This post received KUDOS Expert's post gmatter0913 wrote: Hi Karishma, I have one more doubt on my solution posted earlier. Quote: I tried the problem x-1 < sqrt (7-x) as below: As 7-x is under sqrt, it is +ve. Therefore, 7-x>=0 ; x<=7 ----->(1) x-1 can be -ve or +ve When x-1<=0; x<=1 --------> (2) When x-1>=0; x>=1 --------> (3) As both sides are +ve, we can square both the sides (x-1)^2 < 7-x x^2 -x -6<0 (x-3)(x+2)<0 -2<x<3 ------------>(4) Shouldn't this be 1<=x<3 (as x>=1 is the pre-supposed condition to square them) Most certainly. The only reason I don't care about the values from -2 to 1 is that these values are already covered in the first case. We know they already hold for the inequality. We only get 1 to 3 extra values and that's what we care about. In a stand alone question, the presupposed condition must be satisfied (x <= 7 AND x >= 1) _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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07 Aug 2014, 06:47
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12 Mar 2016, 18:59
Hello from the GMAT Club BumpBot!

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13 Jun 2017, 23:53
Hello from the GMAT Club BumpBot!

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Re: Inequalities and Roots   [#permalink] 13 Jun 2017, 23:53

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