GMAT GLUB top 100 Difficult Math Problems doubts

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wongee: Senior | Next Rank: 100 Posts; Posts: 76; Joined: Sat Jul 28, 2007 1:55 pm; Thanked: 1 times

by wongee » Sat Sep 29, 2007 2:07 pm

Hi, I downloaded and have been reviewing the 100 difficult math problems from this site: firstly, I want to say thats for gathering all the probs together, some are pretty challenging! Are these the difficult probs on the actual gmat, and what is the probability that these types of probs show up on the test?

Also, are there mistakes in any of them? some doubts, PLEASE respond:

1. The sum of the even numbers between 1 and n is 79*80, where n is an odd number, then n=?

Soln: n/2(2a+(n-1)/d) Can someone please explain how this formula came about. I knot about the progression formula, but dont understand this explanation.

=n/2(2*2+(n-1)*2 and this is equal to 79*80 (how can we assume this?)[/b

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Source: Beat The GMAT — Problem Solving | Sat Sep 29, 2007 2:07 pm

samirpandeyit62: Master | Next Rank: 500 Posts; Posts: 460; Joined: Sun Mar 25, 2007 7:42 am; Thanked: 27 times

by samirpandeyit62 » Sun Sep 30, 2007 10:08 am

Hi Wongee
Well the formula is for AP, its good but I use this one

sum of 1st n even nos = n(n+1)

so in 79*80

n=79

Regards
Samir

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wongee: Senior | Next Rank: 100 Posts; Posts: 76; Joined: Sat Jul 28, 2007 1:55 pm; Thanked: 1 times

by wongee » Mon Oct 01, 2007 7:35 pm

hi samir

So, the formula you used is the the for the sum of all even integers?

n(n+1)

If the question asks, what is the sum of all even integers between 1 and 100 OR 20 and 65, how does it work? How do you incorporate the higher end of the spectrum (ie. 65 and 100). thanks, appreciate ur help.

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samirpandeyit62: Master | Next Rank: 500 Posts; Posts: 460; Joined: Sun Mar 25, 2007 7:42 am; Thanked: 27 times

by samirpandeyit62 » Tue Oct 02, 2007 12:39 am

Hi Wongee,
As I mentioned in my post the formula is for 1st n even even nos i.e starting from 2,

you query

How do you incorporate the higher end of the spectrum (ie. 65 and 100). thanks, appreciate ur help.

here we can use AP formula or, on can find sum of even nos upto 100 i.e first 50 even nos- sum of even nos upto 65i.e first 32 evens using n(n+1)

However mostly problems involve finding the first n even nos.

basically IMO one should use these formula's judiciously ,depending upon the spur of the moment (the question).

Regards
Samir

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wongee: Senior | Next Rank: 100 Posts; Posts: 76; Joined: Sat Jul 28, 2007 1:55 pm; Thanked: 1 times

by wongee » Wed Oct 03, 2007 5:35 pm

Thanks! Makes sense, and easier to work with your formula i.e. first n even terms.

However, this AP formula - can you break this down in terms of what is what with a formula. Thanks.

1. The sum of the even numbers between 1 and n is 79*80, where n is an odd number, then n=?

Soln: n/2(2a+(n-1)/d)
a=?
n=?
d=? (common diff?)

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wongee: Senior | Next Rank: 100 Posts; Posts: 76; Joined: Sat Jul 28, 2007 1:55 pm; Thanked: 1 times

by wongee » Sun Oct 07, 2007 4:30 pm

Gracias...The page is in Spanish! Imagine my shock when I see "Progressão aritmética"

No worries, Ill look for the correct Url thanks.

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singh_amit19: Junior | Next Rank: 30 Posts; Posts: 14; Joined: Mon Aug 06, 2007 9:50 pm

Re: GMAT GLUB top 100 Difficult Math Problems doubts

by singh_amit19 » Sun Oct 07, 2007 11:11 pm

wongee wrote:Hi, I downloaded and have been reviewing the 100 difficult math problems from this site: firstly, I want to say thats for gathering all the probs together, some are pretty challenging! Are these the difficult probs on the actual gmat, and what is the probability that these types of probs show up on the test?

Also, are there mistakes in any of them? some doubts, PLEASE respond:

1. The sum of the even numbers between 1 and n is 79*80, where n is an odd number, then n=?

Soln: n/2(2a+(n-1)/d) Can someone please explain how this formula came about. I knot about the progression formula, but dont understand this explanation.

=n/2(2*2+(n-1)*2 and this is equal to 79*80 (how can we assume this?)[/b

Can you please help me with the link??? I am looking for some good Quant practice material like 100 difficult problems!

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wongee: Senior | Next Rank: 100 Posts; Posts: 76; Joined: Sat Jul 28, 2007 1:55 pm; Thanked: 1 times

by wongee » Thu Oct 11, 2007 11:24 am

Here's the link to this and a lot more!

https://www.beatthegmat.com/wiki/doku.ph ... _resources

Under Gmat Math!

Good Luck.

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