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M02-21

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Re: M02-21  [#permalink]

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New post 25 Oct 2017, 00:10
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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Re: M02-21  [#permalink]

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New post 25 Oct 2017, 00:18
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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Re: M02-21  [#permalink]

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New post 11 Feb 2018, 05:03
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?
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New post 11 Feb 2018, 07:44
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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;
\(\sqrt[4]{16} = 2\), 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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Re: M02-21  [#permalink]

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New post 11 Feb 2018, 09:38
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;
\(\sqrt[4]{16} = 2\), 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.
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Re M02-21  [#permalink]

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New post 31 Mar 2018, 07:56
I think this is a high-quality question and I agree with explanation.
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Re M02-21  [#permalink]

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New post 07 Sep 2018, 21:29
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.
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New post 08 Sep 2018, 01:31
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?
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
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Re: M02-21 &nbs [#permalink] 08 Sep 2018, 01:31

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