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GMAT Coordinate Geometry (Data Sufficiency)
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Author:  Robert.Delane [ Sat Mar 03, 2012 3:12 pm ]
Post subject:  GMAT Coordinate Geometry (Data Sufficiency)

What are the coordinates of point A in the rectangular coordinate system?
(1) A is 2 units away from (0, 4).
(2) A is 3 units away from (0, 0).

A. Statement (1) BY ITSELF is sufficient to answer the question, but statement (2) by itself is not.
B. Statement (2) BY ITSELF is sufficient to answer the question, but statement (1) by itself is not.
C. Statements (1) and (2) TAKEN TOGETHER are sufficient to answer the question, even though NEITHER statement BY ITSELF is sufficient.
D. Either statement BY ITSELF is sufficient to answer the question.
E. Statements (1) and (2) TAKEN TOGETHER are NOT sufficient to answer the question, meaning that further information would be needed to answer the question.

(E) All the points that lie at some specific distance from a given one are situated on the circle, which center is the given point and the radius is the distance. Therefore each statement by itself defines a circle (infinitively many points). So each statement by itself is NOT sufficient.
If we use the both statements together, we have two circles. Point A belongs to each one of them. Two circles can cross each other in one or two points. (Two circles can also have no common points at all, but it is NOT our case). The distance between the centers, (0, 0) and (0, 4), is √(0² + 4²) = 4. The sum of the radii is 5. If you draw a segment, which connects these two centers, it’s length will be 4. The circle, which center is (0, 0), will cross the segment 3 units away from (0, 0). The circle, which center is (0, 4), will cross the segment 2 units away from (0, 4). It’s clear, that the circles cross each other in exactly TWO different points. So statements (1) and (2) taken together are NOT sufficient to answer the question. The correct answer is E.
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How can you tell from the question that you are referring to circles?

Author:  Gennadiy [ Sat Mar 03, 2012 3:26 pm ]
Post subject:  Re: t.3, qt.37: circles, coordinate geometry, data sufficien

Quote:
How can you tell from the question that you are referring to circles?
What is a circle?
It has its center and all points of the circle are on the fixed distance from that center. The distance is called a radius.

So when a statement tells us "A is 2 units away from (0, 4)" it means that A is some point of the circle, which center is (0, 4) and the radius is 2.

Author:  dave [ Tue Sep 25, 2012 5:39 am ]
Post subject:  Re: GMAT Coordinate Geometry (Data Sufficiency)

Why is the sum of the radii 5? Which radii are you refering to? Thanks.

Author:  Gennadiy [ Tue Sep 25, 2012 6:31 am ]
Post subject:  Re: GMAT Coordinate Geometry (Data Sufficiency)

questioner wrote:
Why is the sum of the radii 5? Which radii are you refering to? Thanks.
Statement (1) defines a circle with the radius 2 and (0, 4) as its center.
Image
So the point A can be any point on the red circle.

Statement (2) defines a circle with the radius 3 and (0, 0) as its center.
Image
So the point A can be any point on the blue circle.

When we combine the two statements we get
Image
So A can be any of the two green points (intersection of the circles). Thus even combined the statements are NOT sufficient.

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circle2.gif [5.03 KiB]
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circles-combined.gif [7.51 KiB]
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circle1.gif [4.86 KiB]
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