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Math Expert V
Joined: 02 Sep 2009
Posts: 58340

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Terabyte wrote:
What do you guys think about the variable approach from Math Revolution? I think that in some cases it is better to use this approach, but in most cases - not. And if you are aiming for, at least, Q50, variable approach should not be your friend

In short: not a fan of that approach at all.
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Manager  B
Joined: 31 Oct 2016
Posts: 105

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Bunuel wrote:
Terabyte wrote:
What do you guys think about the variable approach from Math Revolution? I think that in some cases it is better to use this approach, but in most cases - not. And if you are aiming for, at least, Q50, variable approach should not be your friend

In short: not a fan of that approach at all.

Agree with you. I attached one file that I prepared comparing answers from GMAT Official Guide and answers from Math Revolution courses. From first 23 only 52% has correct answers. So, for how long they will cheat GMAT-takers? Waste of money and time
>> !!!

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Intern  B
Joined: 13 Oct 2017
Posts: 38

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Hi Bunuel,

I got the correct answer but I'd like clarity on statement 2:

(x-1)^2 = 16
I square rooted both sides to get:
x-1 = 4...then eventually x=5

The crux of my question is this...I got x=4 because the question stated that x is a positive integer.

If the question did not state that x was a positive integer, I would've gone with two solutions: x-1=4 and x-1=-4...giving x=5 and x=(-3) respectively.

Would that have been the correct approach if the question did not specify that x was a positive integer? I'm slightly confused because I think I remember you saying that in gmat land...they only take the positive of a root...so whether or not the question specified x= a positive integer...I still would've had to have taken the positive root?
Math Expert V
Joined: 02 Sep 2009
Posts: 58340

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1
ttaiwo wrote:
Hi Bunuel,

I got the correct answer but I'd like clarity on statement 2:

(x-1)^2 = 16
I square rooted both sides to get:
x-1 = 4...then eventually x=5

The crux of my question is this...I got x=4 because the question stated that x is a positive integer.

If the question did not state that x was a positive integer, I would've gone with two solutions: x-1=4 and x-1=-4...giving x=5 and x=(-3) respectively.

Would that have been the correct approach if the question did not specify that x was a positive integer? I'm slightly confused because I think I remember you saying that in gmat land...they only take the positive of a root...so whether or not the question specified x= a positive integer...I still would've had to have taken the positive root?

(x-1)^2 = 16

x - 1 = 4 or x - 1 = -4

x = 5 or x = -3

When the GMAT provides the square root sign for an even root, such as a square root, fourth root, etc. then the only accepted answer is the positive root. That is:

$$\sqrt{9} = 3$$, NOT +3 or -3;
$$\sqrt{16} = 2$$, NOT +2 or -2;

Notice that in contrast, the equation $$x^2 = 9$$ has TWO solutions, +3 and -3. Because $$x^2 = 9$$ means that $$x =-\sqrt{9}=-3$$ or $$x=\sqrt{9}=3$$.
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Intern  B
Joined: 13 Oct 2017
Posts: 38

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Bunuel wrote:
ttaiwo wrote:
Hi Bunuel,

I got the correct answer but I'd like clarity on statement 2:

(x-1)^2 = 16
I square rooted both sides to get:
x-1 = 4...then eventually x=5

The crux of my question is this...I got x=4 because the question stated that x is a positive integer.

If the question did not state that x was a positive integer, I would've gone with two solutions: x-1=4 and x-1=-4...giving x=5 and x=(-3) respectively.

Would that have been the correct approach if the question did not specify that x was a positive integer? I'm slightly confused because I think I remember you saying that in gmat land...they only take the positive of a root...so whether or not the question specified x= a positive integer...I still would've had to have taken the positive root?

(x-1)^2 = 16

x - 1 = 4 or x - 1 = -4

x = 5 or x = -3

When the GMAT provides the square root sign for an even root, such as a square root, fourth root, etc. then the only accepted answer is the positive root. That is:

$$\sqrt{9} = 3$$, NOT +3 or -3;
$$\sqrt{16} = 2$$, NOT +2 or -2;

Notice that in contrast, the equation $$x^2 = 9$$ has TWO solutions, +3 and -3. Because $$x^2 = 9$$ means that $$x =-\sqrt{9}=-3$$ or $$x=\sqrt{9}=3$$.

Thanks that's much clearer now.
Intern  B
Joined: 20 Mar 2018
Posts: 1

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I think this is a high-quality question and I agree with explanation.
Intern  B
Joined: 08 Dec 2017
Posts: 3
Location: India
Concentration: Marketing, Strategy
GMAT 1: 620 Q48 V27 GRE 1: Q168 V150 GPA: 3.9
WE: Marketing (Education)

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I think this is a poor-quality question. When Y is greater than 1, y^2 is always positive and x is a positive integer so the equation is always positive for any value of X. Statement one is enough to answer the question.
Math Expert V
Joined: 02 Sep 2009
Posts: 58340

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KSANDEEPREDDY wrote:
I think this is a poor-quality question. When Y is greater than 1, y^2 is always positive and x is a positive integer so the equation is always positive for any value of X. Statement one is enough to answer the question.

The question is fine. You just did not read it correctly. The question asks whether y^2(x^3−x+1) > 75? not whether y^2(x^3−x+1) > 0?
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Manager  S
Joined: 09 Jun 2018
Posts: 91
GMAT 1: 610 Q42 V33 GMAT 2: 620 Q40 V35 GMAT 3: 660 Q41 V40 GPA: 3.32

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I fell for the "x is a positive integer" mistake. Great question!
Intern  B
Joined: 09 Apr 2018
Posts: 1

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Dont know y is number or integer. Therefore, only B is not enough

Posted from my mobile device Re M02-21   [#permalink] 16 Jun 2019, 06:29

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