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If arc PQR above is a semicircle, what is the length of

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If arc PQR above is a semicircle, what is the length of [#permalink] New post 13 Dec 2012, 06:14
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If arc PQR above is a semicircle, what is the length of diameter PR ?

(1) a = 4
(2) b = 1
[Reveal] Spoiler: OA
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Re: If arc PQR above is a semicircle, what is the length of [#permalink] New post 13 Dec 2012, 06:16
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If arc PQR above is a semicircle, what is the length of diameter PR?

You should know the following properties to solve this question:
A right triangle inscribed in a circle must have its hypotenuse as the diameter of the circle. The reverse is also true: if the diameter of the circle is also the triangle’s hypotenuse, then that triangle is a right triangle.

So, as given that PR is a diameter then angle PQR is a right angle.

Perpendicular to the hypotenuse will always divide the triangle into two triangles with the same properties as the original triangle.

Thus, the perpendicular QT divides right triangle PQR into two similar triangles PQT and QRT (which are also similar to big triangle PQR). Now, in these three triangles the ratio of the corresponding sides will be equal (corresponding sides are the sides opposite the same angles). For example: QR/PR=QT/PQ=TR/QR. This property (sometimes along with Pythagoras) will give us the following: if we know ANY 2 values from PR, PQ, QR, PT, QT, TR then we'll be able to find other 4. We are given that QT=2 thus to find PR we need to know the length of any other line segment.

Also in such kind of triangles might be useful to equate the areas to find the length of some line segment, for example area of PQR=1/2*QT*PR=1/2*QP*QR (for more check: triangles-106177.html, geometry-problem-106009.html, mgmat-ds-help-94037.html, help-108776.html)

(1) a = 4. Sufficient.

(2) b = 1. Sufficient.

Answer: D.
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Re: If arc PQR above is a semicircle, what is the length of [#permalink] New post 18 Dec 2012, 03:58
Walkabout wrote:
Attachment:
The attachment Semicircle.png is no longer available
If arc PQR above is a semicircle, what is the length of diameter PR ?

(1) a = 4
(2) b= 1


Better you learn some relations which hold true for such triangle.
One which has 2 right angles as here in attachment.
As per that

4 = a*b
a+b=?
Now.
A) a = 4 so b =1
B) b=1 so a = 4

OA D
Attachments

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Re: If arc PQR above is a semicircle, what is the length of [#permalink] New post 22 Dec 2012, 05:04
Expert's post
Walkabout wrote:
Attachment:
The attachment Semicircle2.png is no longer available
If arc PQR above is a semicircle, what is the length of diameter PR ?

(1) a = 4
(2) b= 1


Another approach that could be implemented in thsi question is:
Since there is a perpendicular drawn to the hypotenuese, therefore the two triangles that are formed must be similar to each other and to the larger one.

So if one side of a triangle reduces by a certain ratio, the other side must also reduce.
In the diagram attached, if one considers any of the statement then he will be able to find out the other side.
Consider statement 1) a=4

Look into the diagram. In the middle triangle, "a" or PI=4. We are given with the fact that IQ=2. Now in the smallest triangle, the corresponding side of PI=IQ. IQ=2. Therefore the factor with which PI has reduced is 2. Therefore other side must also reduce by the same factor. Hence IR=1.
Sufficient

Statement 2)
b=1. "b" is the corresponding side of IQ. So IQ , in the middle traingle, has reduced by a factor of 2. In the smallest triangle IQ=2. Therefore PI must be 4.
Sufficient.
Attachments

geometry solution.png
geometry solution.png [ 13.23 KiB | Viewed 3612 times ]


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Re: If arc PQR above is a semicircle, what is the length of [#permalink] New post 22 Dec 2012, 05:05
Bunnel can you explain the below part little elaboarately

For example: QR/PR=QT/PQ=TR/QR. This property (sometimes along with Pythagoras) will give us the following: if we know ANY 2 values from PR, PQ, QR, PT, QT, TR then we'll be able to find other 4. We are given that QT=2 thus to find PR we need to know the length of any other line segment.

I really dont understand the concept
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Re: If arc PQR above is a semicircle, what is the length of [#permalink] New post 14 Jan 2013, 10:38
Marcab wrote:
Walkabout wrote:
Attachment:
Semicircle2.png
If arc PQR above is a semicircle, what is the length of diameter PR ?

(1) a = 4
(2) b= 1


Another approach that could be implemented in thsi question is:
Since there is a perpendicular drawn to the hypotenuese, therefore the two triangles that are formed must be similar to each other and to the larger one.

So if one side of a triangle reduces by a certain ratio, the other side must also reduce.
In the diagram attached, if one considers any of the statement then he will be able to find out the other side.
Consider statement 1) a=4

Look into the diagram. In the middle triangle, "a" or PI=4. We are given with the fact that IQ=2. Now in the smallest triangle, the corresponding side of PI=IQ. IQ=2. Therefore the factor with which PI has reduced is 2. Therefore other side must also reduce by the same factor. Hence IR=1.
Sufficient

Statement 2)
b=1. "b" is the corresponding side of IQ. So IQ , in the middle traingle, has reduced by a factor of 2. In the smallest triangle IQ=2. Therefore PI must be 4.
Sufficient.



Little complex for me... don u think bunuel's method is easier?
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Re: If arc PQR above is a semicircle, what is the length of [#permalink] New post 06 Mar 2014, 09:16
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Re: If arc PQR above is a semicircle, what is the length of [#permalink] New post 29 Mar 2014, 02:55
Can anybody explain why the area of the triangle PQR : Which is a right angle triangle in the figure above at Q, is not 1/2*QR*PQ ? Assuming this triangle was drawn without the semi-circle and if I slightly redraw the triangle keeping the base at QR, and PQ becomes the height and PR is the hypotenuse, then isnt the area of the triangle 1/2*QR*PQ?

Why is it that in these type of triangles which are drawn in such manner, that the hypotenuse is the base, the height is drawn from one vertex to another and then area is calculated?

This question is just to douse this silly doubt lingering in my head.
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Re: If arc PQR above is a semicircle, what is the length of [#permalink] New post 29 Mar 2014, 02:58
Expert's post
sudeeptasahu29 wrote:
Can anybody explain why the area of the triangle PQR : Which is a right angle triangle in the figure above at Q, is not 1/2*QR*PQ ? Assuming this triangle was drawn without the semi-circle and if I slightly redraw the triangle keeping the base at QR, and PQ becomes the height and PR is the hypotenuse, then isnt the area of the triangle 1/2*QR*PQ?

Why is it that in these type of triangles which are drawn in such manner, that the hypotenuse is the base, the height is drawn from one vertex to another and then area is calculated?

This question is just to douse this silly doubt lingering in my head.


The area of triangle PQR IS 1/2*PQ*QR but it's ALSO 1/2*PR*QT.
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NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: If arc PQR above is a semicircle, what is the length of [#permalink] New post 06 May 2014, 21:21
Another alternative and simpler solution is shown in the video, at 2:00.
Re: If arc PQR above is a semicircle, what is the length of   [#permalink] 06 May 2014, 21:21
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