It is currently 19 Oct 2017, 17:26

### 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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# 1/ If the operation # is defined for all integers a and b by

Author Message
SVP
Joined: 05 Jul 2006
Posts: 1750

Kudos [?]: 430 [0], given: 49

1/ If the operation # is defined for all integers a and b by [#permalink]

### Show Tags

15 Oct 2006, 09:41
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

1/ If the operation # is defined for all integers a and b by
a#b = a + b - ab which of the followin statements must be true for all integers a,b, and c?

A) a#b= b#a
B) a#0= a
C) (a#b)#C = a#(b#c)

it is abvious that A,B are true lets check three

a#b = a+b-ab , b#c = b+c - bc

(a+b-ab)#c = a+b-ab+c - ac-bc+abc

(b+c - bc)#a = a+b+c -bc - ab-ac+abc

thus my answer is A,B ONLY

Kudos [?]: 430 [0], given: 49

Manager
Joined: 02 Oct 2006
Posts: 54

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

Location: Paris

### Show Tags

15 Oct 2006, 10:12
First, thanks for seperating the posts...
sorry, next time will be careful!!

In fact for this question, the answer is A,B a,d C!!!
The question was on GMATprep so I only have the answer and not the explanations...

Zz

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

SVP
Joined: 05 Jul 2006
Posts: 1750

Kudos [?]: 430 [0], given: 49

### Show Tags

15 Oct 2006, 10:35

Kudos [?]: 430 [0], given: 49

Manager
Joined: 27 Mar 2008
Posts: 53

Kudos [?]: 6 [0], given: 0

### Show Tags

29 Mar 2008, 19:25
a#b = a + b - ab

We already know that (A) and (B) are true.

C) (a#b)#C = a#(b#c)

LS
= (a+b-ab) + c - (a+b-ab)*c
= a + b - ab + c - ac - bc + abc
= a + b + c - ab - ac - bc + abc (reordering)

RS
= a + (b+c-bc) - a(b+c-bc)
= a + b + c - bc -ab -ac + abc
= a + b + c - ab - ac - bc + abc

Therefore, LS=RS and all of A, B, and C are true.

I think it's the confusion caused when multiplying all the terms. We just gotta be extra careful, which is tough when we're rushed to do these questions in 2 minutes!

Kudos [?]: 6 [0], given: 0

Re: Zamzim separate posts   [#permalink] 29 Mar 2008, 19:25
Display posts from previous: Sort by