Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » Manhattan GMAT CAT Math


 Page 1 of 1 [ 4 posts ] 



  Print view Print view Previous topic Previous topic | Next topic Next topic
 

Geometry-data sufficiency

Author Message
Lavir7
 Post subject: Geometry-data sufficiency
 Post Posted: Thu Jul 09, 2009 4:50 am 
Offline
Students


Posts: 1
A Triangle ABC is given with 2 angles : x,y are angles lying across AC and AB ,respectively.

Is triangle ABC isosceles?

1/ X does not equal Y
2/ AB=2BC

The answer given by MGMAT is (C) - both assumptions are needed.
( 1-If x and Y are different it tells us nothing about the third angle AND2- if AB=2BC, BC could still equal AC the )

I think there's a mistake in the answer and the answer is really (B)- meaning that the second piece of information suffices. If AB=2BC and BC+AC must be >AB (triangle rules) , than AC+BC>2BC and AC>BC. i.e. they are not equal and the triangle ABC cannot an isoceles if all sides are different!

Have I reached the wrong conclusion?
Top 
RonPurewal
 Post subject: Re: Geometry-data sufficiency
 Post Posted: Mon Jul 13, 2009 7:38 am 
Offline
ManhattanGMAT Staff


Posts: 7146
Lavir7 wrote:
A Triangle ABC is given with 2 angles : x,y are angles lying across AC and AB ,respectively.

Is triangle ABC isosceles?

1/ X does not equal Y
2/ AB=2BC

The answer given by MGMAT is (C) - both assumptions are needed.
( 1-If x and Y are different it tells us nothing about the third angle AND2- if AB=2BC, BC could still equal AC the )

I think there's a mistake in the answer and the answer is really (B)- meaning that the second piece of information suffices. If AB=2BC and BC+AC must be >AB (triangle rules) , than AC+BC>2BC and AC>BC. i.e. they are not equal and the triangle ABC cannot an isoceles if all sides are different!

Have I reached the wrong conclusion?


yes.

you are correct in concluding that AC must be greater than BC, but nothing precludes the possibility that AC = AB (i.e., both of them = 2BC). if that is the case, then the triangle will be isosceles.

that's a nice job of manipulating the triangle inequalities, though.
Top 
adityaj123
 Post subject: Re: Geometry-data sufficiency
 Post Posted: Sun Nov 07, 2010 8:11 am 
Offline
Students


Posts: 2
Does the questions with answer NO come in real GMAT exam??
Top 
jnelson0612
 Post subject: Re: Geometry-data sufficiency
 Post Posted: Sun Nov 07, 2010 5:33 pm 
Offline
ManhattanGMAT Staff


Posts: 1857
adityaj123 wrote:
Does the questions with answer NO come in real GMAT exam??


adityaj123, definitely! Based on the information in a statement, you absolutely can get an answer of NO for the question, and NO would be sufficient. Yes, we have seen many such instances of that on the real GMAT.

_________________
Jamie Nelson
ManhattanGMAT Instructor
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 4 posts ] 





Who is online

Users browsing this forum: No registered users and 0 guests

 
 

 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to: