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Geometry Problem ? angle that bisects triangle in two half..

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by rommysingh » Wed Aug 05, 2015 4:04 pm
If AD is 6, and ADC is a right angle, what is the area of triangular region ABC?

(1) Angle ABD = 60°

(2) AC = 12

when ADC is a right angle triangle..it is implied that ADB is also a right angle triangle..
the answer should be A or D to say the least but the answer is C?
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by MartyMurray » Wed Aug 05, 2015 4:52 pm
rommysingh wrote:If AD is 6, and ADC is a right angle, what is the area of triangular region ABC?

(1) Angle ABD = 60°

(2) AC = 12

when ADC is a right angle triangle..it is implied that ADB is also a right angle triangle..
the answer should be A or D to say the least but the answer is C?
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Yes, because while you know from Statement 1 that angle ABD = 60°, you don't know the measure of angle ACD, or anything else that you can use to determine all the dimensions of triangle ABC, or those of triangle ACD. For all you know the length of AC is 100. With the measure of AC given in Statement 2, you gain the length of another side of right triangle ACD, and that along with the information given in the question and in Statement 1 is enough to calculate everything.

Did you assume that AD bisects angle BAC? That is not stated anywhere.
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by rommysingh » Wed Aug 05, 2015 5:33 pm
Thanks for the reply Marty, I assumed it bisects the triangle hence coz the figure was so misleading.
But i get it now.
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by Jim@StratusPrep » Thu Aug 06, 2015 12:26 pm
One way that I think about geometry figures is by thinking of what points are 'fixed' and which are not. In each of the statements, one of the endpoints of the triangle can be shifted to create different sized triangles.
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by Brent@GMATPrepNow » Thu Aug 06, 2015 12:40 pm
Jim@StratusPrep wrote:One way that I think about geometry figures is by thinking of what points are 'fixed' and which are not. In each of the statements, one of the endpoints of the triangle can be shifted to create different sized triangles.
Jim brings up a very useful strategy for solving geometry DS questions. For these questions, we're typically checking to see whether the statements "lock" a particular angle or length into having just one value.

This concept is discussed in our free video: https://www.gmatprepnow.com/module/gmat- ... cy?id=1103

Cheers,
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