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Source: — Data Sufficiency |
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by vishubn » Sat Dec 20, 2008 8:54 am
IMO B

if yes I will post the sikn

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by logitech » Sat Dec 20, 2008 10:06 am
1) CB = 5.6

So we can find CD.

AC x CD = CB^2 ( put in a flash card )

we can find AD

AD = AC + CD

AD x CD = 7 X AB

so we can find AB - suf

2) CBD = 60, so we can find both CD and CB and the rest of the calculation is as in statement 1 , so SUF

I would choose D

It is a bad ass question!
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by Brent@GMATPrepNow » Sat Dec 20, 2008 11:24 am
Nice work, Logitech!

One important piece is to recognize that triangles ACB and BCD are similar triangles.

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by logitech » Sat Dec 20, 2008 11:50 am
Brent Hanneson wrote:Nice work, Logitech!

One important piece is to recognize that triangles ACB and BCD are similar triangles.

Check out my DS question (cars and students) - I'd love to see your assessment.
Brent, these are wonderful questions. I already checked the other question. Smart Smart Smart!! We are so lucky to have you here.
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by vittalgmat » Mon Dec 22, 2008 4:09 am
logitech wrote:1) CB = 5.6

So we can find CD.

AC x CD = CB^2 ( put in a flash card ) <<<-------???

we can find AD

AD = AC + CD

AD x CD = 7 X AB

so we can find AB - suf

2) CBD = 60, so we can find both CD and CB and the rest of the calculation is as in statement 1 , so SUF

I would choose D

It is a bad ass question!
Hi Logitech,
Can u pls explain the theory behind this formula above ??

Thanks
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by amitabhprasad » Mon Dec 22, 2008 12:13 pm
I believe that's because
triangle ACB and BCD are similar
AB/BD=BC/CD=AC/BC
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Whats the question

by CrackGMAC » Wed Dec 24, 2008 7:39 am
Brent I can't see the question at-all :!:
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by sumatitandon » Sat Dec 27, 2008 11:48 pm
Pls help,
will u pls explain ur working , i fail to understand
thx
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by aroon7 » Sun Dec 28, 2008 12:08 am
Triangles ABD and BCD are similar, because:
1. Side BD is common to both
2. Angle BDC is common to both
3. Both are right angled triangles. i.e Angle ABD and Angle BCD are equal

hence we can write:
(BC/AB) = (BD/CD)

from (1): BC = 5.6
we can find out CD pythagores theorem.
so in the eqn (BC/AB) = (BD/CD), there is only one unknown, which is AB.
hence this is sufficient

from (2): angle CBD = 60 deg
so angle BDA = 30
hence the sides in triangle ABD are in ratio: root (3), 1 and 2
since we know one of the sides (BD = 7) we can find the other two.
hence this alone is sufficient too.

Ans is D
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by Ian Stewart » Sun Dec 28, 2008 9:20 am
The diagram in the question is actually related to one of the most famous diagrams in geometry - the one Euclid used to prove the Pythagorean Theorem. The early proofs of Pythagoras relied on the fact that you get three similar triangles when you draw, as in the diagram above, a height connecting the hypotenuse to the vertex with the 90 degree angle. Because we get similar triangles in such a picture, we really don't need much additional information to find all the lengths here, as logitech observed above. One other useful takeaway: it's generally worthwhile looking for similar triangles in any diagram where one triangle contains another, since when this happens, the two triangles are certain to share at least one angle.

My only suggestion, from a question design point of view, would be to remove the contradiction between the two statements - for example, to make Statement 1 read "CB = 3.5", so that it's logically possible to consider both statements together (using the first statement, each triangle is similar to a 3-4-5 triangle, while using the second, each triangle is a 30-60-90 triangle). Of course, the real GMAT always ensures the two statements are logically consistent.
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by Brent@GMATPrepNow » Sun Dec 28, 2008 10:43 am
Nice call, Ian.
I forgot to ensure consistency.
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