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shamikba1981
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Quadrilateral ABCD is a rhombus

by shamikba1981 Mon Nov 08, 2010 12:12 am

Quadrilateral ABCD is a rhombus and points C, D, and E are on the same line. Is quadrilateral ABDE a rhombus?

(1) The measure of angle BCD is 60 degrees.
(2) AE is parallel to BD


Image
Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
Both statements TOGETHER are sufficient, but NEITHER one ALONE is sufficient.
EACH statement ALONE is sufficient.
Statements (1) and (2) TOGETHER are NOT sufficient.



1) INSUFFICIENT:

Draw segment BD. Since BC = CD (because ABCD is a rhombus), Triangle BCD is an isosceles triangle. Since angle BCD = 60, the remaining angles in the triangle must also equal 60 degrees and Triangle BCD is actually an equilateral triangle. Since BD must also be equal to AB and AD, Triangle ABD is also an equilateral triangle and all of its angles measure 60 degrees. Since CD is parallel to AB and DE is an extension of line CD, we know that DE is also parallel to AB. Using AD as a transversal, we know the measure of angle ADE = 60. However, we know nothing about angles DAE or AED and with only one pair of opposite sides parallel we cannot conclude that quadrilateral ABDE is a rhombus.

(2) INSUFFICIENT:

Knowing that AE is parallel to BD allows us to conclude that alternate interior angles DAE and ADB are congruent. For the same reasons stated above, we know that DE is parallel to AB, and with two pairs of opposite sides parallel we know we have a parallelogram. But we have no further evidence that quadrilateral ABDE is a rhombus.

(1) and (2) SUFFICIENT:

If we combine both statements we can conclude that drawing segment BD creates three equilateral triangles. We already know ABDE is a parallelogram, and since all of its sides are congruent we can conclude that it is a rhombus.




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I think the explanation is incorrect. it should be (2) is SUFFICIENT
This is because DAE is equal to ADB. But also because DE is parallel to AB angle BAE is equal to andle ADE. Also since BA = AD (since ABCD is a rhombus) Hence triangle BAD is congruent to traingle ADE.
Therefore ABDE is a rhombus
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Re: Quadrilateral ABCD is a rhombus

by jnelson0612 Thu Nov 11, 2010 7:01 pm

shamikba,
You've done well with identifying angles that are equal. However, for ABDE to be a rhombus we also must know that all the sides must be equal. The problem is that we really don't know how long AB and DE are. Let's pretend that AE and BD are both 5. AB and DE could both be 5, or they could be 6. We really don't know. You can redraw the figure and lengthen AB and DE to help you see this.

Thank you,
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Re: Quadrilateral ABCD is a rhombus

by frank_le Thu Oct 20, 2011 7:13 pm

N/a Deleted my response...
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Re: Quadrilateral ABCD is a rhombus

by jnelson0612 Sun Nov 20, 2011 12:35 am

frank_le Wrote:N/a Deleted my response...


Okay! :-)
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Re: Quadrilateral ABCD is a rhombus

by lsegal Mon Nov 28, 2011 5:13 pm

Sorry, I'm confused how a 3 equilateral triangles make a rhombus?

Thanks in advance.
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Re: Quadrilateral ABCD is a rhombus

by tim Tue Dec 13, 2011 2:47 am

Now i’m confused. Did someone say that three equilateral triangles made a rhombus?
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Re: Quadrilateral ABCD is a rhombus

by NanoBotZ44 Tue Jun 26, 2012 10:46 pm

Question :

Since ABCD is a Rhombus, AB = AD
Also since AB // CE, <BAD = <ADE.
Triangles ABD and DEA congruent.
Therefore BD = AE.

So AE//BD is sufficient to prove that ABDE is a RHOMBUS.

Correct me if I am wrong .. please.
Andy
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Re: Quadrilateral ABCD is a rhombus

by tim Wed Jun 27, 2012 1:48 am

well, you know what the answer is, so you can deduce that you must be wrong about something. ideally you would work through the information in the problem to figure this out as you are reviewing the problem. you are correct that BD = AE. it is also true that AB = DE. this is not enough to prove that we have a rhombus though; we would only know that if all four sides are equal..
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rte.sushil
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Re: Quadrilateral ABCD is a rhombus

by rte.sushil Tue Nov 20, 2012 11:53 am

I have idea of the all the figures to find out whether it is rectangle, square, rhombus , parallelogram but whenever i encounter some problem it takes me long time to solve it as i need to think about that figure from all possible shapes, thus taking a long time in solving or end up in marking wrong answer. Conclusion is : Efficiency is poor for such question.

Is there any link where i can learn on tips so that i can get rid of above long process? Or any link where it is directly mentioned now that conditions this shape can be other shape also.e.g. rhombus can be parallelogram if ....something like this....or some helpful links.

Thanks!!
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Re: Quadrilateral ABCD is a rhombus

by jlucero Tue Nov 20, 2012 5:21 pm

I like the table in the middle of this page:

http://www.mathsisfun.com/quadrilaterals.html
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rte.sushil
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Re: Quadrilateral ABCD is a rhombus

by rte.sushil Thu Dec 06, 2012 12:39 pm

jlucero Wrote:I like the table in the middle of this page:

http://www.mathsisfun.com/quadrilaterals.html



Great! very clearly explained.
Thanks!!
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Re: Quadrilateral ABCD is a rhombus

by jlucero Fri Dec 07, 2012 12:51 pm

You're welcome
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Re: Quadrilateral ABCD is a rhombus

by navjeetmann Fri Jul 05, 2013 2:39 am

tim Wrote:well, you know what the answer is, so you can deduce that you must be wrong about something. ideally you would work through the information in the problem to figure this out as you are reviewing the problem. you are correct that BD = AE. it is also true that AB = DE. this is not enough to prove that we have a rhombus though; we would only know that if all four sides are equal..
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Re: Quadrilateral ABCD is a rhombus

by tim Fri Jul 05, 2013 2:23 pm

Let us know if there are any further questions on this one.
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Re: Quadrilateral ABCD is a rhombus

by ammarjavaid007 Thu Mar 06, 2014 12:04 pm

Hey Tim,

So what I'm getting from this is that if opposite angles of a four sided shape are the same, we can't just assume that opposite sides are the same (since we need both to prove that it is a rhombus). Does this also work the other way around? As in, if opposite sides are the same length (and parallel), we can't assume that opposite angles will be the same.

Correct me if I'm wrong.

Thanks.