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cool_jonny009
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DS-Geometry GMATPrep 19 Feb 2006, 19:58
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Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!

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giddi77
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DS-Geometry GMATPrep 19 Feb 2006, 21:48
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Initially selected B. I solved it again and found it to be D. This is definitely a tough one!

From (1)

See attached picture,

x + (2x+60) +60 = 180
x = 20

From (2)
BCO = CBO = 40 (triangle BCO is isoceles with OB = OC
Angle ABO = 180 - CBO = 140

Also triangle ABO is isoceles with AB = BO
BAO = BOA = x

2x+140 = 180
x = 20

Hence D.
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cool_jonny009
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DS-Geometry GMATPrep 20 Feb 2006, 00:01
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yup OA is D ...man you are damm good... 8-)

can you please explain why the angle BCO = 2 OAB i.e why x=2x

in this equation
x + (2x+60) +60 = 180
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cool_jonny009
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DS-Geometry GMATPrep 20 Feb 2006, 00:06
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giddi I got it.. after staring on the figure you have drawn :idea: :idea:
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giddi77
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DS-Geometry GMATPrep 20 Feb 2006, 00:19
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cool_jonny009
yup OA is D ...man you are damm good... 8-)

can you please explain why the angle BCO = 2 OAB i.e why x=2x

in this equation
x + (2x+60) +60 = 180
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Because BO = OC = radius
=> Trainge CBO is an isoceles triangle
=>Angle BOC = COB

Now CBO is sum of 2 angles each of which is x. Reason for this is:

There are 2 parallel lines.
1. The diameter
2. The line drawn from B which is parallel to the diameter (say BE)

These two are cut by OB. Angles on the either side of a line (transversal) that cuts two paralle lines are equal. So EBO = BOC

Now CBE = CAO again because diameter is paralle to BE

HTH.

Good luck for your test!! I am sure you will do well.
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MBAlad
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DS-Geometry GMATPrep 20 Feb 2006, 19:33
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Am almost there but not seeing how you get "x + (2x+60) +60 = 180"

Which line are you calculating the 180 angle for?

Where are you getting 2x+60 from?

Help me please, my brain hurts!!
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MBAlad
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DS-Geometry GMATPrep 20 Feb 2006, 20:37
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Come on guys - GMAT in 2 days - could really do with a bit of help here.

Thanks!
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DS-Geometry GMATPrep 20 Feb 2006, 20:43
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It doesn't have to be so bad. Angle COD = 60 degrees, therefore angle AOC = 120 degrees.

We're given another angle in triangle AOC, which is BCO, that is, 40 degrees.

For angles in triangle AOC to total 180 degrees, implies:

180 - angle AOC - angle BCO = angle BAO.

180 - 120 - 40 = 20 degrees = angle BAO.

That's it! (assuming ABC is a straight line)
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MBAlad
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DS-Geometry GMATPrep 20 Feb 2006, 20:52
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Ahhhhhhhhhhhhh - Got it. Thanks.

Can go to bed now (is 3.45am in UK and have to work tomorrow)!!!



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This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
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Thank you for understanding, and happy exploring!
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