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RonPurewal
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Re: If angle BAD is a right angle, what is the length of side BD

by RonPurewal Wed Aug 05, 2015 1:25 am

AliasgharA585 wrote:Thanks Ron for the explanation... :)


you're welcome.
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Re: If angle BAD is a right angle, what is the length of side BD

by EnriqueR905 Fri Mar 25, 2016 8:52 pm

Hello, i still do not understand why it is not A. It seeems like since if we look at the image from the question we can clearly observe that we have a right angle and then statement 1 says that line ac is perpendicular,doesn´t that mean that the 90 degrees angle is cut in half and because we have a perpendicular line,we have then a 45-45-90 triangle which would make statement 1 sufficient by itself?.
Could someone help me clarify this?

Thank You.
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Re: If angle BAD is a right angle, what is the length of side BD

by tim Sat Mar 26, 2016 5:11 pm

We still don't have a diagram. We will be glad to help you once someone posts the original diagram from the problem.
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Re: If angle BAD is a right angle, what is the length of side BD

by EnriqueR905 Sat Mar 26, 2016 9:42 pm

Hello, i am sorry i could not upload the image directly,but here it is: http://gmatclub.com/forum/download/file.php?id=13493
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Re: If angle BAD is a right angle, what is the length of side BD

by tim Sat Mar 26, 2016 9:46 pm

Thanks. Your mistake is assuming that angle A is always bisected by altitude AC. That occurs if and only if the large triangle is isosceles, so you used an assumption that the triangle was isosceles to prove that it is isosceles.
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Re: If angle BAD is a right angle, what is the length of side BD

by ZadiaH581 Mon Apr 11, 2016 3:34 pm

Hello,

I know this is an older post, but I was wondering since Statement 1 says that AC is perpendicular to BD, don't we automatically know that that BC and DC are the same length, since perpendicular bisectors are known to divide a line segment exactly in half? If that is the case, should A not be sufficient to know that the two triangles are similar? Also, does AC divide angle BAD also in half? Or does this rule not happy to perpendicular lines in triangles?

Thank you for your help in advance.
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Re: If angle BAD is a right angle, what is the length of side BD

by RonPurewal Fri Apr 15, 2016 10:04 pm

sure, but, statement 1 doesn't say "perpendicular bisector"... it just says "perpendicular".
so, there's no reason why C would have to be the midpoint of BD.
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Re: If angle BAD is a right angle, what is the length of side BD

by LukeT323 Sat Dec 24, 2016 12:14 am

tim wrote:Thanks. Your mistake is assuming that angle A is always bisected by altitude AC. That occurs if and only if the large triangle is isosceles, so you used an assumption that the triangle was isosceles to prove that it is isosceles.


Shouldn't I assume that AC splits angle BAD into two 45 degree angles?
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Re: If angle BAD is a right angle, what is the length of side BD

by RonPurewal Mon Jan 02, 2017 2:03 am

LukeT323 wrote:Shouldn't I assume that AC splits angle BAD into two 45 degree angles?


...most definitely not. if the right angle is formed by 2 sides of significantly different lengths, then those angles won't be anywhere even close to 45° each.

...as in this figure
http://www.mathplanet.com/Oldsite/media ... 99x300.jpg

it should be absolutely clear that angles ABD and DBC in that figure are nowhere close to 45°. (angle ABD is much bigger than 45°, and angle DBC is much smaller than 45°.)