GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 17 Oct 2019, 21:01

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

If x+y=a and x-y=b, then 2xy=? a) (a^2 - b^2)/2 b)

Author Message
Intern
Joined: 27 Jan 2005
Posts: 15
If x+y=a and x-y=b, then 2xy=? a) (a^2 - b^2)/2 b)  [#permalink]

Show Tags

27 Jan 2005, 21:47
00:00

Difficulty:

(N/A)

Question Stats:

86% (01:26) correct 14% (00:00) wrong based on 42 sessions

HideShow timer Statistics

14. If x+y=a and x-y=b, then 2xy=?

a) (a^2 - b^2)/2

b) (b^2 - a^2)/2

c) (a - b )/2
d) (ab)/2
e) (a^2 + b^2)/2

There must be a FAST way to do this. Pluggin' in takes too long.

--== Message from the GMAT Club Team ==--

THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION.
This discussion does not meet community quality standards. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Intern
Joined: 26 Dec 2004
Posts: 4

Show Tags

27 Jan 2005, 22:06
1
Square both the sides and then substract

(x + y)^2 = a^2
- (x - y)^2 = b^2

4xy = a^2 - b^2
2xy = a^2 - b^2/2
Director
Joined: 07 Jun 2004
Posts: 555
Location: PA

Show Tags

28 Jan 2005, 05:13
when u see these kind of formula just do algeraic maniuplation not plug in tht will work fast
Director
Joined: 31 Aug 2004
Posts: 509

Show Tags

28 Jan 2005, 07:37
Just notice that you need a positive product "2xy" so the sign before b whatever the power must be negative : this reduces the choice between A and C.

Then between A and C you need a power to have square terms (a-b does not lead in any way to xy...)

In this way it takes no more than 20 sec.
Manager
Joined: 25 Oct 2004
Posts: 204

Show Tags

28 Jan 2005, 08:03
Great tip twixt. I would not have thought about going this way. I went about solving it. Even that took around 30-40 sec...... But good tip.
Intern
Joined: 27 Jan 2005
Posts: 15

Show Tags

28 Jan 2005, 13:26
thanks to you all!
Manager
Joined: 28 Jan 2005
Posts: 93

Show Tags

30 Jan 2005, 07:27
excellent tip. I think the hard part for most of us is keeping our cool when hit with such questions, and trying to understand what is going in instead of jumping into the calculations. Any tips on how to overcome these?
Intern
Joined: 11 Jan 2005
Posts: 47
Location: Mexico City

Show Tags

01 Feb 2005, 10:42
My biggest problems are often these types of questions. i see them and my mind freaks out.

I have learned that often they want you to square it first or multiply by 1 in the form of x^2y/y or something.

Those are my two cents but i could use some advice myself.
Manager
Joined: 11 Jan 2005
Posts: 85

Show Tags

01 Feb 2005, 18:19
my tip ?

not to crack the formula as it ll take tremendous time.
here : Narrow the possible solutions then replace
c & d, definitively not
e, tougher but also obvious

we are left with
a or b

plug the assumptions and simplify.
you got it ! A

--== Message from the GMAT Club Team ==--

THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION.
This discussion does not meet community quality standards. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Non-Human User
Joined: 09 Sep 2013
Posts: 13240
Re: If x+y=a and x-y=b, then 2xy=? a) (a^2 - b^2)/2 b)  [#permalink]

Show Tags

27 Dec 2018, 10:07
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
Re: If x+y=a and x-y=b, then 2xy=? a) (a^2 - b^2)/2 b)   [#permalink] 27 Dec 2018, 10:07
Display posts from previous: Sort by