Author 
Message 
TAGS:

Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 48074

Question Stats:
58% (00:30) correct 42% (00:29) wrong based on 253 sessions
HideShow timer Statistics



Math Expert
Joined: 02 Sep 2009
Posts: 48074

Re M1920
[#permalink]
Show Tags
16 Sep 2014, 01:06



Math Expert
Joined: 02 Sep 2009
Posts: 48074

Re: M1920
[#permalink]
Show Tags
21 Nov 2014, 04:51



Current Student
Joined: 04 Jul 2014
Posts: 302
Location: India
GMAT 1: 640 Q47 V31 GMAT 2: 640 Q44 V34 GMAT 3: 710 Q49 V37
GPA: 3.58
WE: Analyst (Accounting)

Re: M1920
[#permalink]
Show Tags
21 Nov 2014, 05:05
Bunuel wrote: The point is that x  3 = 3  x. Both indicate the distance between x and 3. So, I understand that both of our answers are correct and that both these values won't appear as options in a single question.
_________________
Cheers!!
JA If you like my post, let me know. Give me a kudos!



Math Expert
Joined: 02 Sep 2009
Posts: 48074

Re: M1920
[#permalink]
Show Tags
21 Nov 2014, 05:07



Math Expert
Joined: 02 Sep 2009
Posts: 48074

Re: M1920
[#permalink]
Show Tags
24 Jul 2015, 07:51
riyazgilani wrote: Bunuel wrote: Official Solution:
Which of the following always equals \(\sqrt{9 + x^2  6x}\) ?
A. \(x  3\) B. \(3 + x\) C. \(3  x\) D. \(3 + x\) E. \(3  x\)
\(\sqrt{9 + x^2  6x} = \sqrt{(3  x)^2} = 3  x\) (by the definition of a square root).
Answer: C i answered choice a : x3 i took the the expression as x^26x+9 > (x3)^2. no my understanding is that on GMAT when some expression is under root sign, only positive root is considered...? is it correct. if it is so, the we need not take mod value...? pls help Exactly because the square root function cannot give negative result the answer is 3x, an absolute value of 3x, which also cannot be negative. MUST KNOW: \(\sqrt{x^2}=x\):The point here is that since square root function can not give negative result then \(\sqrt{some \ expression}\geq{0}\). So \(\sqrt{x^2}\geq{0}\). But what does \(\sqrt{x^2}\) equal to? Let's consider following examples: If \(x=5\) > \(\sqrt{x^2}=\sqrt{25}=5=x=positive\); If \(x=5\) > \(\sqrt{x^2}=\sqrt{25}=5=x=positive\). So we got that: \(\sqrt{x^2}=x\), if \(x\geq{0}\); \(\sqrt{x^2}=x\), if \(x<0\). What function does exactly the same thing? The absolute value function: \(x=x\), if \(x\geq{0}\) and \(x=x\), if \(x<0\). That is why \(\sqrt{x^2}=x\). I'd advice to go through basics and only after to practice questions:
_________________
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: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
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? Extrahard Quant Tests with Brilliant Analytics



Manager
Joined: 03 Dec 2013
Posts: 71
Location: United States (HI)
GPA: 3.56

Re M1920
[#permalink]
Show Tags
22 Aug 2016, 14:02
I think this is a highquality question and I agree with explanation.



Current Student
Joined: 18 Jun 2015
Posts: 41

Re: M1920
[#permalink]
Show Tags
12 Sep 2016, 15:23
After solving this question I got the result as X3 and I answered it as X3, And got this wrong. Thanks for giving the link which clears the basic of Absolute value including the result:
AB = BA
Which is clearly applicable in this question provided options.



Manager
Joined: 14 Oct 2012
Posts: 175

Bunuel wrote: riyazgilani wrote: Bunuel wrote: Official Solution:
Which of the following always equals \(\sqrt{9 + x^2  6x}\) ?
A. \(x  3\) B. \(3 + x\) C. \(3  x\) D. \(3 + x\) E. \(3  x\)
\(\sqrt{9 + x^2  6x} = \sqrt{(3  x)^2} = 3  x\) (by the definition of a square root).
Answer: C i answered choice a : x3 i took the the expression as x^26x+9 > (x3)^2. no my understanding is that on GMAT when some expression is under root sign, only positive root is considered...? is it correct. if it is so, the we need not take mod value...? pls help Exactly because the square root function cannot give negative result the answer is 3x, an absolute value of 3x, which also cannot be negative. MUST KNOW: \(\sqrt{x^2}=x\):The point here is that since square root function can not give negative result then \(\sqrt{some \ expression}\geq{0}\). So \(\sqrt{x^2}\geq{0}\). But what does \(\sqrt{x^2}\) equal to? Let's consider following examples: If \(x=5\) > \(\sqrt{x^2}=\sqrt{25}=5=x=positive\); If \(x=5\) > \(\sqrt{x^2}=\sqrt{25}=5=x=positive\). So we got that: \(\sqrt{x^2}=x\), if \(x\geq{0}\); \(\sqrt{x^2}=x\), if \(x<0\).What function does exactly the same thing? The absolute value function: \(x=x\), if \(x\geq{0}\) and \(x=x\), if \(x<0\). That is why \(\sqrt{x^2}=x\). I'd advice to go through basics and only after to practice questions: Hello Bunuel, VyshakI thought the equation was as follows: \(\sqrt{x^2}=x\), if \(x>{0}\); \(\sqrt{x^2}=x\), if \(x<={0}\). Am i wrong? Please correct me if so. Thanks



Math Expert
Joined: 02 Sep 2009
Posts: 48074

Re: M1920
[#permalink]
Show Tags
30 Mar 2017, 22:01



SVP
Joined: 26 Mar 2013
Posts: 1785

Re: M1920
[#permalink]
Show Tags
31 Mar 2017, 10:00
Dear Bunuel,
The answer choices CAN"T have the following answers in their list: \(3  x\) and \(x  3\) Because the following rule:
\(3  x\) = \(x  3\)
Am I right?
Thanks in advance



Manager
Joined: 14 Oct 2012
Posts: 175

Re: M1920
[#permalink]
Show Tags
31 Mar 2017, 15:07
Bunuel wrote: manishtank1988 wrote: Hello Bunuel, VyshakI thought the equation was as follows: \(\sqrt{x^2}=x\), if \(x>{0}\); \(\sqrt{x^2}=x\), if \(x<={0}\). Am i wrong? Please correct me if so. Thanks You can put = sign in any of the two because \(\sqrt{0^2}=0=0\). Understood thanks...



Manager
Joined: 09 Nov 2016
Posts: 64
Location: India

Re: M1920
[#permalink]
Show Tags
21 Aug 2017, 05:28
This question is based on the principle of \(\sqrt{x^2}=x\) Hence its \(x3=3x\) Straight away C. Press Kudos id this helps!



Intern
Joined: 13 Oct 2017
Posts: 39

Re: M1920
[#permalink]
Show Tags
09 Feb 2018, 07:42
Hi Bunuel,
I used the plugin method...but still a little confused as to why C was correct...I did the following:
made x=2...then...√9+412 = √1 as my goal.
I then plugged in the two in place of x in the answer choices...to see if I got a figure that corresponded with my goal of √1.
A) 23= 1 NO B) 3+2= 5 NO C) [32] = [1] MAYBE D) [3+2] = [5] NO E) 32 = 1 MAYBE
I originally chose E because the goal of √1, seems to result in 1 and 1 only when one finds the square root. However, upon a second look...is [1] another way of saying √1, thus making C the correct answer?



Math Expert
Joined: 02 Sep 2009
Posts: 48074

Re: M1920
[#permalink]
Show Tags
09 Feb 2018, 07:49
ttaiwo wrote: Hi Bunuel,
I used the plugin method...but still a little confused as to why C was correct...I did the following:
made x=2...then...√9+412 = √1 as my goal.
I then plugged in the two in place of x in the answer choices...to see if I got a figure that corresponded with my goal of √1.
A) 23= 1 NO B) 3+2= 5 NO C) [32] = [1] MAYBE D) [3+2] = [5] NO E) 32 = 1 MAYBE
I originally chose E because the goal of √1, seems to result in 1 and 1 only when one finds the square root. However, upon a second look...is [1] another way of saying √1, thus making C the correct answer? C is correct because it's generally true, always true for ALL values of x. As for your approach: for plugin method it might happen that for some particular number(s) more than one option may give "correct" answer. In this case just pick some other numbers and check again these "correct" options only. For example, check x = 4.
_________________
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: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
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? Extrahard Quant Tests with Brilliant Analytics



Intern
Joined: 13 Oct 2017
Posts: 39

Re: M1920
[#permalink]
Show Tags
09 Feb 2018, 08:46
Hi Bunuel,
Yup, I've plugged in x=4 and got √1 as the goal (so the goal remained unchanged)
A) 1, NO B) 7, NO C) [1], MAYBE D) [7], NO E) 1, NO
So in this case, C is conclusively the case because it is always true for all values of x...
My question remains though...is √1 always [1], and is that also the case for √1?
Thanks again,
Tosin



Math Expert
Joined: 02 Sep 2009
Posts: 48074

Re: M1920
[#permalink]
Show Tags
09 Feb 2018, 08:51
ttaiwo wrote: Hi Bunuel,
Yup, I've plugged in x=4 and got √1 as the goal (so the goal remained unchanged)
A) 1, NO B) 7, NO C) [1], MAYBE D) [7], NO E) 1, NO
So in this case, C is conclusively the case because it is always true for all values of x...
My question remains though...is √1 always [1], and is that also the case for √1?
Thanks again,
Tosin \(\sqrt{1}=1\) only. \(\sqrt{1}\) is an imaginary number and since all numbers on the GMAT are real numbers you don't need to worry about that at all.. 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 \(x=\sqrt{9}=3\).
_________________
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: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
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? Extrahard Quant Tests with Brilliant Analytics



Study Buddy Forum Moderator
Joined: 04 Sep 2016
Posts: 1169
Location: India
WE: Engineering (Other)

Re: M1920
[#permalink]
Show Tags
29 Jul 2018, 09:09
BunuelI agree this is a pure gem of a question. I can not believe my eyes that so many people fell for (A) just thinking it is x3 instead of actually x3 . +1 Kudos !!
_________________
It's the journey that brings us happiness not the destination.



Intern
Joined: 19 Feb 2018
Posts: 10

Re: M1920
[#permalink]
Show Tags
14 Aug 2018, 20:49
Bunuel wrote: Official Solution:
Which of the following always equals \(\sqrt{9 + x^2  6x}\) ?
A. \(x  3\) B. \(3 + x\) C. \(3  x\) D. \(3 + x\) E. \(3  x\)
\(\sqrt{9 + x^2  6x} = \sqrt{(3  x)^2} = 3  x\) (by the definition of a square root).
Answer: C I completely disagree with OA. While this is true that when the equation is \(x^2 = 25\) then the value of x can be either 5 or +5. However, this rule is not applicable to the given question. If you have equation something like this \((x2)^2 = 25\) then you will have to solve for either \((x2)=5\) or \((x2)=5\). Note you will treat both eq(1) and eq(2) separately. However, if a question has an equation then you have to treat that equation as it is, assuming that it is one of two equation that has been given to you to solve and you have to solve it as it is without adding new details to it. GMAT will refrain from adding any ambiguity to question and the same information is stated in Manhattan's book as well. This question is of poor quality.



Math Expert
Joined: 02 Sep 2009
Posts: 48074

Re: M1920
[#permalink]
Show Tags
14 Aug 2018, 23:51
harsh8686 wrote: Bunuel wrote: Official Solution:
Which of the following always equals \(\sqrt{9 + x^2  6x}\) ?
A. \(x  3\) B. \(3 + x\) C. \(3  x\) D. \(3 + x\) E. \(3  x\)
\(\sqrt{9 + x^2  6x} = \sqrt{(3  x)^2} = 3  x\) (by the definition of a square root).
Answer: C I completely disagree with OA. While this is true that when the equation is \(x^2 = 25\) then the value of x can be either 5 or +5. However, this rule is not applicable to the given question. If you have equation something like this \((x2)^2 = 25\) then you will have to solve for either \((x2)=5\) or \((x2)=5\). Note you will treat both eq(1) and eq(2) separately. However, if a question has an equation then you have to treat that equation as it is, assuming that it is one of two equation that has been given to you to solve and you have to solve it as it is without adding new details to it. GMAT will refrain from adding any ambiguity to question and the same information is stated in Manhattan's book as well. This question is of poor quality. You completely missed the main idea of the question: the square root function can not give negative result. Let me assure you that the question is 100% correct. You'll benefit greatly if you read and study the whole thread CAREFULLY.
_________________
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: GMAT Math Book  Triangles  Polygons  Coordinate Geometry  Factorials  Circles  Number Theory  Remainders; 8. Overlapping Sets  PDF of Math Book; 10. Remainders  GMAT Prep Software Analysis  SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS)  Tricky questions from previous years.
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? Extrahard Quant Tests with Brilliant Analytics










