Grockit- PS-must be true

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arora007: Community Manager; Posts: 1048; Joined: Mon Aug 17, 2009 3:26 amFollowed by: 27 members; Location: India; Thanked: 51 times; GMAT Score:670

by arora007 » Mon Dec 13, 2010 6:45 am

If a and b are different positive integers and a + b = a(a + b), then which of the following must be true?

I. a = 1

II. b = 1

III. a < b

A) I only
B) II only
C) III only
D) I and II
E) I and III

I liked this one, If u dont rush you will get it right

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Source: Beat The GMAT — Problem Solving | Mon Dec 13, 2010 6:45 am

frank1: Legendary Member; Posts: 537; Joined: Fri Jan 22, 2010 10:06 pmFollowed by: 1 member; Thanked: 14 times

by frank1 » Mon Dec 13, 2010 7:58 am

I think it should be E

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N:Dure: Master | Next Rank: 500 Posts; Posts: 181; Joined: Sat Oct 16, 2010 1:57 pm; Thanked: 4 times

by N:Dure » Mon Dec 13, 2010 8:22 am

E

If a= 1 and b then has to be different so = 2 a+b = 3 = a(a+b) = 3

if b=1 and a=2 a+b=3 a(a+b)= 6

Then a =1 works and b has to be different so it must be bigger ie. a<b

mj78ind: Master | Next Rank: 500 Posts; Posts: 265; Joined: Mon Dec 28, 2009 9:45 pmFollowed by: 2 members; Thanked: 26 times; GMAT Score:760

by mj78ind » Mon Dec 13, 2010 9:34 am

arora007 wrote:If a and b are different positive integers and a + b = a(a + b), then which of the following must be true?

I. a = 1

II. b = 1

III. a < b

A) I only
B) II only
C) III only
D) I and II
E) I and III

I liked this one, If u dont rush you will get it right

Ok am a bit rusty

(a+b) = a((a+b)

=> (a-1)(a+b)=0

This is only possible if a= 1. Now a and b are different if a= 1, then b has to be greater than 1 since both are different positive integers. Hence E

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arora007: Community Manager; Posts: 1048; Joined: Mon Aug 17, 2009 3:26 amFollowed by: 27 members; Location: India; Thanked: 51 times; GMAT Score:670

by arora007 » Mon Dec 13, 2010 10:00 pm

yeah..its E... hope nobody chose A like I did when I had about 30 seconds for the problem!

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