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Post subject: GRE Geometry (Select One) Posted: Tue Jul 20, 2010 11:28 am 

Joined: Sun May 30, 2010 3:15 am Posts: 424

In the figure above, lines L and P are parallel. The segment AD is the same length as the segment DC; AD is parallel to BC. If the length of AD is equal to 4, and angle ADC is equal to 60º, what is the area of ABCD?
A. 4√2 B. 4√3 C. 8√2 D. 8√3 E. 16√2
(D) AB is parallel to DC and AD is parallel to BC, so ABCD is parallelogram. We know how to find the area of a parallelogram we multiply the height of parallelogram by its base length.
We are told that the angle ADC is a 60º angle. If we draw an altitude from A straight down and perpendicular to line P, the length of that altitude will be equal to the height of the rhombus. Moreover, it forms a 603090 triangle, so we can easily find its length. Since the hypotenuse of that triangle is equal to 4, the second longest side must be equal to 2√3 (since the proportions for the triangle run x, x√3, and 2x).
To find the area of a rhombus, multiply the height of the rhombus by its base length: 4 × 2√3 = 8√3, or answer choice (A).  The sum of the interior angles should be (n – 2) × 180. So the sum of the interior angles for ABCD = 360⁰. How is it determined that the traingle formed from A is a 306090 triangle?


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Gennadiy

Post subject: Re: math (test 1, question 9): geometry Posted: Tue Jul 20, 2010 11:39 am 

Joined: Sun May 30, 2010 2:23 am Posts: 498

We determine that triangle EDA is a 906030 triangle because:
angle ADC is 60 degrees (we know that from the question statement) angle AED is 90 degrees because AE is height of parallelogram ABCD angle EAD is 30 degrees since it equals 180⁰ – angle AED  angle ADC = 180⁰ – 90⁰ – 60⁰ = 30⁰


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questioner

Post subject: Re: math (test 1, question 9): geometry Posted: Thu Aug 12, 2010 8:06 am 

Joined: Sun May 30, 2010 3:15 am Posts: 424

How did you deduce that the parallelogram formed its special case (i.e., a rhombus)? Given the information from the question and the fact that the figure is not drawn to scale, I find it hard to deduce that the figure is a rhombus.


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Gennadiy

Post subject: Re: math (test 1, question 9): geometry Posted: Thu Aug 12, 2010 8:50 am 

Joined: Sun May 30, 2010 2:23 am Posts: 498

Question statement tells us: "In the figure above, lines L and P are parallel. ... AD is parallel to BC." So we know that ABCD is a parallelogram. As any parallelogram its opposite sides are equal. So AB = DC and AD = BC.
Besides, "the segment AD is the same length as the segment DC". So now all 4 sides are equal. Therefore ABCD is rhombus.


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questioner

Post subject: Re: math (test 1, question 9): geometry Posted: Fri Feb 24, 2012 7:00 pm 

Joined: Sun May 30, 2010 3:15 am Posts: 424

I think the answer should be 16√3. As area of each triangle of the parallelogram would be 1/2 × 4 × 4 × sin(60⁰) = 8√3.


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Gennadiy

Post subject: Re: math (test 1, question 9): geometry Posted: Fri Feb 24, 2012 7:03 pm 

Joined: Sun May 30, 2010 2:23 am Posts: 498


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