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If AD is 6 and ADC is a right angle, what is the area of triangular re

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If AD is 6 and ADC is a right angle, what is the area of triangular re  [#permalink]

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New post 16 Oct 2017, 02:46
zanaik89 wrote:
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Given: \(AD=6\). Question: \(area_{ABC}=\frac{1}{2}*AD*BC=\frac{1}{2}*6*(BD+DC)=3(BD+DC)=?\)

(1) Angle ABD = 60 --> triangle ABD is 30-60-90 triangle, so the sides are in ratio \(1:\sqrt{3}:2\) --> as AD=6 (larger leg opposite 60 degrees angle) then \(BD=\frac{6}{\sqrt{3}}\) (smallest leg opposite 30 degrees angle). But we still don't know DC. Not sufficient.

(2) AC=12, we can find DC. But we still don't know BD. Not sufficient.

(1)+(2) We know both BD and DC, hence we can find area. Sufficient.

Answer: C.


Hi Bunuel

Cant we assume that that AD bisects BC into two?

Please clear my doubt thanks[/quote]


So u mean that since it isnt given that triangle ABC is an isoceles triangle or equilateral triangle, we can not consider AD as a perpendicular bisector right?
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New post 31 Aug 2018, 08:58
If AD is 6 and ADC is a right angle, what is the area of triangular region ABC?[/b]

Given: \(AD=6\). Question: \(area_{ABC}=\frac{1}{2}*AD*BC=\frac{1}{2}*6*(BD+DC)=3(BD+DC)=?\)

(1) Angle ABD = 60 --> triangle ABD is 30-60-90 triangle, so the sides are in ratio \(1:\sqrt{3}:2\) --> as AD=6 (larger leg opposite 60 degrees angle) then \(BD=\frac{6}{\sqrt{3}}\) (smallest leg opposite 30 degrees angle). But we still don't know DC. Not sufficient.

(2) AC=12, we can find DC. But we still don't know BD. Not sufficient.

(1)+(2) We know both BD and DC, hence we can find the area. Sufficient.

Answer: C.

Bunuel - My doubt is if a triangle like this which has a 30-60-90 angles, shouldn't the the triangle ADC should also be 30-60-90 making triangle ABC an equilateral triangle? because an equilateral triangle can be formed by joining two identical triangles, each with 30-60-90 angles.
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Re: If AD is 6 and ADC is a right angle, what is the area of triangular re  [#permalink]

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New post 09 Jul 2019, 07:17
Bunuel VeritasKarishma The stem says that ADC is a right angle. That means AD is perpendicular to DC. Why are we assuming that B, D and C are in a straight line?
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New post 10 Jul 2019, 06:21
GittinGud wrote:
Bunuel VeritasKarishma The stem says that ADC is a right angle. That means AD is perpendicular to DC. Why are we assuming that B, D and C are in a straight line?


BC is a straight line and D lies on it. Note sure why you think right triangle ADC messes that up.
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Re: If AD is 6 and ADC is a right angle, what is the area of triangular re  [#permalink]

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New post 10 Jul 2019, 08:47
VeritasKarishma We have no information in the stem to decide whether BC is a straight line. Why are we assuming it is?
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New post 10 Jul 2019, 11:12
VeritasKarishma wrote:
AccipiterQ wrote:
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I thought if you dropped a line down from a triangle vertex and it formed a right angle on the opposite side then that line bisected the side? So in this case if you know what BD is then you know what DC is?



To figure out whether it holds, why don't you try drawing some extreme figures, say, something like this:
Attachment:
Ques3.jpg


Will this be true in this case?
When will it be true? When the triangle is equilateral, sure. Also when the triangle is isosceles if the equal sides form the angle from which the altitude is dropped.

Don't put your faith in the figure given. It may be just one of the many possibilities or may be somewhat misleading.


Hi! How do we know that the ADC is also a 90 30 60 triangle?
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Re: If AD is 6 and ADC is a right angle, what is the area of triangular re  [#permalink]

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New post 10 Jul 2019, 14:15
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GittinGud wrote:
VeritasKarishma We have no information in the stem to decide whether BC is a straight line. Why are we assuming it is?


I am pasting the response that Bunuel posted earlier. Hope that will help in clearing the confusion up.

OG13, page 272:
A figure accompanying a data sufficiency problem will conform to the information given in the question but will not necessarily conform to the additional information given in statements (1) and (2).
Lines shown as straight can be assumed to be straight and lines that appear jagged can also be assumed to be straight.
You may assume that the positions of points, angles, regions, and so forth exist in the order shown and that angle measures are greater than zero degrees.
All figures lie in a plane unless otherwise indicated.

OG13, page 150:
Figures: A figure accompanying a problem solving question is intended to provide information useful in solving the problem. Figures are drawn as accurately as possible. Exceptions will be clearly noted. Lines shown as straight are straight, and lines that appear jagged are also straight. The positions of points, angles, regions, etc., exist in the order shown, and angle measures are greater than zero. All figures lie in a plane unless otherwise indicated.
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Re: If AD is 6 and ADC is a right angle, what is the area of triangular re  [#permalink]

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New post 12 Jul 2019, 00:05
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GittinGud wrote:
VeritasKarishma We have no information in the stem to decide whether BC is a straight line. Why are we assuming it is?


A figure may not be to scale but lines that look straight are certainly straight. Also, points and angles are in the order in which they are given.
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Re: If AD is 6 and ADC is a right angle, what is the area of triangular re   [#permalink] 12 Jul 2019, 00:05

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