Author 
Message 
Intern
Joined: 30 Dec 2003
Posts: 33
Location: Danbury

Geometry Circle [#permalink]
Show Tags
08 May 2008, 05:08
This topic is locked. If you want to discuss this question please repost it in the respective forum. Can anyone pls help.
Attachments
circle.doc [78.5 KiB]
Downloaded 219 times



Current Student
Joined: 27 Mar 2008
Posts: 415
Schools: Kellogg Class of 2011

Re: Geometry Circle [#permalink]
Show Tags
08 May 2008, 07:41
Since O is the center of a semicircle and B,C,D lie on the semicircle:
BO = OC = OD
We know AB = OC (from question stem) so, BO = OC = OD =AB
We know that in a triangle if 2 sides are equal in length, then they have the same angle
In triangle AOB, since AB = OB, angle(BAO) = angle (BOA) In triangle BOC, since OB = OC, angle(OBC) = angle (OCB)
(1) angle (COD) = 60, Thus, angle (COA) = 180  60 = 120. Not sufficient (2) angle (BCO) = 40 which means angle (CBO) = 40 [They are equal angles] So, angle(BOC) = 1804040 = 100 Not sufficient
When you combine the 2 statements, angle(COA) = 120 and angle (BOC) = 100 So, angle (BOA) = 20. We already know angle (BOA) = angle (BAO)
Thus, both (1) and (2) together yield angle (BAO) = 20.
Answer is C.



Current Student
Joined: 28 Dec 2004
Posts: 3357
Location: New York City
Schools: Wharton'11 HBS'12

Re: Geometry Circle [#permalink]
Show Tags
08 May 2008, 08:05
i get B..
lets see..i know triangle BCO is issoceles..if I know BCO..then i also know CBO.. if know CBO then i know that 180CBO=ABO..
i know triangle ABO is isso too..so AOB=BAO..
i think B should be the answer..



Manager
Joined: 27 Jun 2007
Posts: 199

Re: Geometry Circle [#permalink]
Show Tags
08 May 2008, 08:53
1. The degree measure of angle COD is 60.
If we are given that COD is 60 degrees, then angle COA must be 120 degrees (180  60). This does not help us with angle BAO that we are trying to find.
Insufficient. Thus, eliminate A & D.
2. The degree measure of angle BCO is 40.
We are given BCO, but this measure alone does not help us with finding BAO. Thus eliminate B.
Trying both together, in triangle ACO, in order to find one angle measurement, you would need 2 angles out of 3. Combining both 1 and 2 together, we are given that. By being provided the measurement of angle COD in 1, you can find COA as 120 degrees. Further, in 2 you are given the measure of BCO, which is 40. Combining the two, the result is 160 degrees (120 + 40 = 160). Subtracting 160 from 180 will give you 20 degrees as the measurement for angle CAO, thus providing enough info. to solve the problem.
Eliminate E.
Answer is C.



Current Student
Joined: 27 Mar 2008
Posts: 415
Schools: Kellogg Class of 2011

Re: Geometry Circle [#permalink]
Show Tags
08 May 2008, 09:00
fresinha12 wrote: i get B..
lets see..i know triangle BCO is issoceles..if I know BCO..then i also know CBO.. if know CBO then i know that 180CBO=ABO..
i know triangle ABO is isso too..so AOB=BAO..
i think B should be the answer.. fresinha, you're right. I completely overlooked that we know ABO =140 based on B and ABO + 2*BAO = 180 Answer should be B



Senior Manager
Joined: 21 Apr 2008
Posts: 488
Schools: Kellogg, MIT, Michigan, Berkeley, Marshall, Mellon

Re: Geometry Circle [#permalink]
Show Tags
11 Nov 2008, 08:55
Hi guys, sorry for coming it up again, but it is not well solved. AO is D Any idea why?
Attachments
geometry.JPG [ 75.97 KiB  Viewed 2387 times ]
_________________
mates, please visit my profile and leave comments http://gmatclub.com/forum/johnlewis1980sprofilefeedbackismorethanwelcome80538.html
I'm not linked to GMAT questions anymore, so, if you need something, please PM me
I'm already focused on my application package
My experience in my second attempt http://gmatclub.com/forum/p544312#p544312 My experience in my third attempt http://gmatclub.com/forum/630q47v28engineernonnativespeakermyexperience78215.html#p588275



CEO
Joined: 17 Nov 2007
Posts: 3584
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth)  Class of 2011

Re: Geometry Circle [#permalink]
Show Tags
11 Nov 2008, 10:04
1. AB=OC=OB > triangles ABO and BOC are isosceles. 2. BAO=BOA, OBC=BCO 3. ABO=180OBC > 1802*BAO=180BCO > BAO=BCO/2 (second condition)4. COD=180AOBBOC=180BAO(1802*BCO)=180BAO180+2*2*BAO=3BAO > BAO=COD/3 (first condition)
_________________
HOT! GMAT TOOLKIT 2 (iOS) / GMAT TOOLKIT (Android)  The OFFICIAL GMAT CLUB PREP APP, a musthave app especially if you aim at 700+  PrepGame



Senior Manager
Joined: 21 Apr 2008
Posts: 488
Schools: Kellogg, MIT, Michigan, Berkeley, Marshall, Mellon

Re: Geometry Circle [#permalink]
Show Tags
16 Nov 2008, 09:55
Thank you so much Walker +1 kudos! Could you explain the way in which you assume that the line CBA is a straight line? That's the only point I miss This problem is really tough Thank you walker wrote: 1. AB=OC=OB > triangles ABO and BOC are isosceles. 2. BAO=BOA, OBC=BCO 3. ABO=180OBC > 1802*BAO=180BCO > BAO=BCO/2 (second condition) 4. COD=180AOBBOC=180BAO(1802*BCO)=180BAO180+2*2*BAO=3BAO > BAO=COD/3 (first condition)
_________________
mates, please visit my profile and leave comments http://gmatclub.com/forum/johnlewis1980sprofilefeedbackismorethanwelcome80538.html
I'm not linked to GMAT questions anymore, so, if you need something, please PM me
I'm already focused on my application package
My experience in my second attempt http://gmatclub.com/forum/p544312#p544312 My experience in my third attempt http://gmatclub.com/forum/630q47v28engineernonnativespeakermyexperience78215.html#p588275



CEO
Joined: 17 Nov 2007
Posts: 3584
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth)  Class of 2011

Re: Geometry Circle [#permalink]
Show Tags
16 Nov 2008, 10:47
JohnLewis1980 wrote: Could you explain the way in which you assume that the line CBA is a straight line? That's the only point I miss
GMAT gives you the figure and at this figure CBA is a line. It is so unusual maybe even unreal that GMAT uses such geometric illusion At least I've never seen such type of ambiguity in real problems.
_________________
HOT! GMAT TOOLKIT 2 (iOS) / GMAT TOOLKIT (Android)  The OFFICIAL GMAT CLUB PREP APP, a musthave app especially if you aim at 700+  PrepGame



Senior Manager
Joined: 21 Apr 2008
Posts: 488
Schools: Kellogg, MIT, Michigan, Berkeley, Marshall, Mellon

Re: Geometry Circle [#permalink]
Show Tags
17 Nov 2008, 03:22
mmm, ok, I see thanks again Walker! walker wrote: JohnLewis1980 wrote: Could you explain the way in which you assume that the line CBA is a straight line? That's the only point I miss
GMAT gives you the figure and at this figure CBA is a line. It is so unusual maybe even unreal that GMAT uses such geometric illusion At least I've never seen such type of ambiguity in real problems.
_________________
mates, please visit my profile and leave comments http://gmatclub.com/forum/johnlewis1980sprofilefeedbackismorethanwelcome80538.html
I'm not linked to GMAT questions anymore, so, if you need something, please PM me
I'm already focused on my application package
My experience in my second attempt http://gmatclub.com/forum/p544312#p544312 My experience in my third attempt http://gmatclub.com/forum/630q47v28engineernonnativespeakermyexperience78215.html#p588275




Re: Geometry Circle
[#permalink]
17 Nov 2008, 03:22






