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# questions including [x]

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Joined: 05 Apr 2012
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questions including [x] [#permalink]  11 Aug 2012, 06:56
does anyone know what [x] means?

eg: If [x] denotes the greatest integer less than or equal to x, is [x] = 0

I'm having trouble understanding what If [x] denotes the greatest integer less than or equal to x means
Manhattan GMAT Instructor
Joined: 05 Jun 2012
Posts: 73
Location: United States
Schools: UCSD (Rady) - Class of 2011
GMAT 1: 760 Q49 V44
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Re: questions including [x] [#permalink]  11 Aug 2012, 07:56
Expert's post
Well it looks to me like [x] denotes the greatest integer less than or equal to x.

("denotes" = "is")

It's a function that the GMAT made up.
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Re: questions including [x] [#permalink]  11 Aug 2012, 08:27
1
KUDOS
rjacobsMGMAT wrote:
Well it looks to me like [x] denotes the greatest integer less than or equal to x.

("denotes" = "is")

It's a function that the GMAT made up.

It's not a function made up by GMAT )
Carl Friedrich Gauss introduced the square bracket notation [x] for the floor function, which indeed, associates to every x the greatest integer less than or equal to x. Much later, in computer science, Iverson introduced different notations for the floor $$\lfloor{x}\rfloor$$ (and ceiling $$\lceil{x}\rceil$$) function. In mathematics both notations are used.
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PhD in Applied Mathematics
Love GMAT Quant questions and running.

Last edited by EvaJager on 11 Aug 2012, 13:10, edited 2 times in total.
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Re: questions including [x] [#permalink]  11 Aug 2012, 08:41
2
KUDOS
Pillah wrote:
does anyone know what [x] means?

eg: If [x] denotes the greatest integer less than or equal to x, is [x] = 0

I'm having trouble understanding what If [x] denotes the greatest integer less than or equal to x means

Every real number can be placed between two consecutive integers, such that $$k\leq{x}<k+1$$.
For every given $$x$$, the integer $$k$$with the above property is unique. By definition, then $$[x] = k$$.
You can always write $$[x]\leq{x}<[x]+1.$$
If $$x=5,$$ $$[5]=5$$. If $$x=5.67$$, $$[x]=5.$$
$$[0]=0$$, $$[0.84]=0$$, but $$[-0.34]=-1.$$

If $$x$$ is an integer, than $$[x]$$ is simply $$x.$$
For non-integer $$x$$, try to visualize the number line. The closest integer to $$x$$ from the left is $$[x].$$
We used to call it the integer part function. For positive numbers, just ignore the decimal part and leave the integer part of the number.
For negative numbers, don't forget to look at the left of your number. $$[-2.7] = -3$$ and not $$-2.$$

So, if $$[x] = 0$$, then certainly $$0\leq{x}<1$$, meaning $$x$$ can be any number between 0 and 1, 0 inclusive, 1 exclusive.
_________________

PhD in Applied Mathematics
Love GMAT Quant questions and running.

Manhattan GMAT Instructor
Joined: 05 Jun 2012
Posts: 73
Location: United States
Schools: UCSD (Rady) - Class of 2011
GMAT 1: 760 Q49 V44
GPA: 3.7
WE: Education (Education)
Followers: 208

Kudos [?]: 97 [0], given: 3

Re: questions including [x] [#permalink]  11 Aug 2012, 20:51
Expert's post
EvaJager wrote:

It's not a function made up by GMAT )
Carl Friedrich Gauss introduced the square bracket notation [x] for the floor function, which indeed, associates to every x the greatest integer less than or equal to x. Much later, in computer science, Iverson introduced different notations for the floor $$\lfloor{x}\rfloor$$ (and ceiling $$\lceil{x}\rceil$$) function. In mathematics both notations are used.

HA that's great, I totally didn't know that! I usually teach my students that this is the equivalent of the RoundDown function on Excel, but I think I like this history lesson better
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Ryan Jacobs | Manhattan GMAT Instructor | San Francisco

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Kudos [?]: 599 [0], given: 43

Re: questions including [x] [#permalink]  11 Aug 2012, 22:10
rjacobsMGMAT wrote:
EvaJager wrote:

It's not a function made up by GMAT )
Carl Friedrich Gauss introduced the square bracket notation [x] for the floor function, which indeed, associates to every x the greatest integer less than or equal to x. Much later, in computer science, Iverson introduced different notations for the floor $$\lfloor{x}\rfloor$$ (and ceiling $$\lceil{x}\rceil$$) function. In mathematics both notations are used.

HA that's great, I totally didn't know that! I usually teach my students that this is the equivalent of the RoundDown function on Excel, but I think I like this history lesson better

I know this is not the place, but I see I can't send you a PM (personal message) through the site. Thank you for the Kudos, but I think I didn't do much to earn it in this case:o)

BTW, I thoroughly enjoyed your post "Winning ugly on the GMAT", Kudos for it!
I cannot deny Agassi's achievements, but my all times favorites is still Sampras.
Although he never won Roland Garros, he had a superb game at the net, which today's player lack so much.
Great message and amazing idea to connect tennis with GMAT!

For those who didn't read the article yet:
http://www.manhattangmat.com/blog/index ... -the-gmat/
_________________

PhD in Applied Mathematics
Love GMAT Quant questions and running.

Last edited by EvaJager on 12 Aug 2012, 07:19, edited 1 time in total.
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Re: questions including [x] [#permalink]  12 Aug 2012, 06:29
Expert's post
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Re: questions including [x] [#permalink]  16 Aug 2012, 22:07
Expert's post
Pillah wrote:
does anyone know what [x] means?

eg: If [x] denotes the greatest integer less than or equal to x, is [x] = 0

I'm having trouble understanding what If [x] denotes the greatest integer less than or equal to x means

When working on Floor or Ceiling functions, a useful technique is to think of the number line.

Say x lies somewhere on the number line. The floor function, often denoted by [x], is the integer closest to x on the left hand side i.e. it is the largest number which is smaller than x (or equal to x if x is an integer). It is easy to comprehend in case of positive numbers. e.g.
[3.4] = 3,
[5.789] = 5 etc
You should be a little more careful while dealing with negative numbers
[-2.45] = -3
This is why the number line perspective helps.
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Re: questions including [x]   [#permalink] 16 Aug 2012, 22:07
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