99,999^2 - 1^2 =? - The Beat The GMAT Forum - Expert GMAT Help & MBA Admissions Advice

99,999^2 - 1^2 =?

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seanceserene: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Fri Feb 26, 2010 8:56 pm; Thanked: 2 times

by seanceserene » Sun Mar 28, 2010 5:44 am

99,999^2 - 1^2 =?

A. 10^10 - 2
B. (10^5 - 2)^2
C. 10^4(10^5 - 2)
D. 10^5(10^4 - 2)
E. 10^5(10^5 - 2)

i am going to take the exam tomorrow, please help with this tricky one
thanks in advance

it is not an "alice in wonderland". it is real! i am going to freak GMAT out!

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Source: Beat The GMAT — Problem Solving | Sun Mar 28, 2010 5:44 am

neoreaves: Master | Next Rank: 500 Posts; Posts: 208; Joined: Sun Sep 28, 2008 12:30 pm; Thanked: 22 times

by neoreaves » Sun Mar 28, 2010 6:01 am

ok first of all best of luck ...just relax when you take the exam ...gmat only tests a few concepts but it tricks you so you cant use them

like in this question all you need to know is

x^2 -1 = (x+1)(x-1)

so 99999^2 - 1^2 = (99999 + 1) (99999 - 1) = 10^5(10^5-2)

So the answer should be E

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pops: Master | Next Rank: 500 Posts; Posts: 148; Joined: Tue Jun 09, 2009 11:06 pm; Thanked: 7 times

by pops » Sun Mar 28, 2010 6:18 am

seanceserene wrote:99,999^2 - 1^2 =?

A. 10^10 - 2
B. (10^5 - 2)^2
C. 10^4(10^5 - 2)
D. 10^5(10^4 - 2)
E. 10^5(10^5 - 2)

i am going to take the exam tomorrow, please help with this tricky one
thanks in advance

Lots of Good Luck and Good Wishes to you !!

Rather than the answer to your problem, you should remember these formulae:
a^2-b^2=(a+b)(a-b)
(a+b)^2=a^2+b^2+2ab
(a-b)^2=a^2+b^2-2ab
a^3-b^3=(a-b)(a^2+b^2+ab)
a^3+b^3=(a+b)(a^2+b^2-ab)

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seanceserene: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Fri Feb 26, 2010 8:56 pm; Thanked: 2 times

by seanceserene » Sun Mar 28, 2010 9:07 pm

neoreaves wrote:ok first of all best of luck ...just relax when you take the exam ...gmat only tests a few concepts but it tricks you so you cant use them

like in this question all you need to know is

x^2 -1 = (x+1)(x-1)

so 99999^2 - 1^2 = (99999 + 1) (99999 - 1) = 10^5(10^5-2)

So the answer should be E

OMG, i mistook the "," for dot.
i thought it is nighty-nine point nine nine nine, which injects me despair
thank you..

it is not an "alice in wonderland". it is real! i am going to freak GMAT out!

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GMAT/MBA Expert

Scott@TargetTestPrep: GMAT Instructor; Posts: 8137; Joined: Sat Apr 25, 2015 10:56 amFollowed by: 29 members; Location: Los Angeles, CA; Thanked: 43 times

by Scott@TargetTestPrep » Tue May 08, 2018 10:36 am

seanceserene wrote:99,999^2 - 1^2 =?

A. 10^10 - 2
B. (10^5 - 2)^2
C. 10^4(10^5 - 2)
D. 10^5(10^4 - 2)
E. 10^5(10^5 - 2)

Using the difference of squares, we have:

(99,999 + 1)(99,999 - 1)

(100,000)(99,998)

(100,000)(100,000 - 2)

10^5(10^5 - 2)

Answer: E

Scott Woodbury-Stewart
Founder and CEO
[email protected]

See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews

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