gmat prep square root and modulus

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by rahulsaxena8 » Sun Aug 08, 2010 8:43 pm
commonsense27 wrote:sqrt(x^2)/x=((x^2)^1/2)/x =x^1-1=x^0=1

It does not matter what is value of x is. Answer is 1. If book is saying otherwise, then book needs to be corrected.

BTW, What is the source of this problem?
Hi,

Source is GMAT Prep.

I hope i don't get a similar question in GMAT. As after seeing this approach i m back "to square 1" , pun is totally intended.

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by selango » Sun Aug 08, 2010 9:38 pm
commonsense27 wrote:sqrt(x^2)/x=((x^2)^1/2)/x =x^1-1=x^0=1

It does not matter what is value of x is. Answer is 1. If book is saying otherwise, then book needs to be corrected.

BTW, What is the source of this problem?
sqrt(x^2)/x=((x^2)^1/2)/x

((x^2)^1/2)/x

Let say x=3

Now there are 2 solutions for this.

((3^3)^1/2)/3=(9)^1/2/3

(9)^1/2/3 can be expressed in 2 ways,

((3^2)1/2)/3 or (((-3)^2)1/2)/3

1 or -1
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by sanju09 » Sun Aug 08, 2010 11:01 pm
rahulsaxena8 wrote:Image
[/img]
The value x^2 is always positive, if x ≠ 0. And a positive number inside √ is taken as positive only. The value x could be either positive or negative, so the positivity constraint forces us to take √x^2 as |x| only. The answer to this question is -1, if x < 0; and it is 1, if x > 0. Henceforth, two answers are possible, and [spoiler]E[/spoiler] covers both.
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by selango » Sun Aug 08, 2010 11:08 pm
Thanks Sanju.

Rahul,

Sorry I created a lot of confusion here.

The important point to be noted is sqrt(x^2) is always positive whatever the sign of x.

If x=6,sqrt(36)/6=6/6=1

If x=-6,sqrt(36)/-6=6/-6=-1
Last edited by selango on Sun Aug 08, 2010 11:12 pm, edited 1 time in total.
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by sanju09 » Sun Aug 08, 2010 11:11 pm
selango wrote:Thanks Sanju.

Rahul,

Sorry I created a lot of confusion here.

The important point to be noted is sqrt(x^2) is always positive whatever the sign of x.

If x=6,sqrt(36)=6/6=1

If x=-6,sqrt(36)/-6=6/-6=-1
now what is this in bold?
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by rahulsaxena8 » Sun Aug 08, 2010 11:47 pm
selango wrote:Thanks Sanju.

Rahul,

Sorry I created a lot of confusion here.

The important point to be noted is sqrt(x^2) is always positive whatever the sign of x.

If x=6,sqrt(36)/6=6/6=1

If x=-6,sqrt(36)/-6=6/-6=-1
Anyways I am glad there are people who actively contribute. I was having tough time understanding why my answer was wrong.

Thanks a lot to everyone who contributed.

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by sanju09 » Mon Aug 09, 2010 12:26 am
rahulsaxena8 wrote:
selango wrote:Thanks Sanju.

Rahul,

Sorry I created a lot of confusion here.

The important point to be noted is sqrt(x^2) is always positive whatever the sign of x.

If x=6,sqrt(36)/6=6/6=1

If x=-6,sqrt(36)/-6=6/-6=-1
Anyways I am glad there are people who actively contribute. I was having tough time understanding why my answer was wrong.

Thanks a lot to everyone who contributed.
Thank you the thanks' stockpile. I can count 4 from you in this single thread so far. Interesting to see Rahul Lakhani not getting one from you.
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by commonsense27 » Mon Aug 09, 2010 8:27 am
I still stand by my answer. If book is saying otherwise, book needs to be corrected.

We should use our fundamentals to arrive at a choice. If everytime we change our fundamentals based on the question, we will be in deep shit.

This is an exponential problem. There is absolutely nothing wrong that I am doing.
Square root cancels the square. And
X^0=1 as long as the x !=0.

ANSWER IS 1.

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by Ian Stewart » Mon Aug 09, 2010 11:36 am
commonsense27 wrote:I still stand by my answer. If book is saying otherwise, book needs to be corrected.

We should use our fundamentals to arrive at a choice. If everytime we change our fundamentals based on the question, we will be in deep shit.

This is an exponential problem. There is absolutely nothing wrong that I am doing.
Square root cancels the square. And
X^0=1 as long as the x !=0.

ANSWER IS 1.
No, sqrt(x^2) is *not* always equal to x. You can see this by plugging in x = -2, say. Then sqrt(x^2) = sqrt(4) = 2, which is not equal to x; it's equal to -x. In general, sqrt(x^2) is *always* equal to |x|, but it is only equal to x if x is zero or greater.

When learning a mathematical rule, it's important to learn when you are allowed to apply it, and when you are not. The rule you are trying to use here, that (x^a)^b = x^(ab), is *not* always true if x is negative. If you don't want to take my word for it, you can read the 'Real powers of positive numbers' section here:

en.wikipedia.org/wiki/Exponentiation#Identities_and_properties

The GMAT does this all the time - tests if you understand the exceptions to the familiar algebraic rules you have learned. Often it's just as important to know when you *can't* apply a rule as it is to know when you can.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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