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In the xy-plane, point (r, s) lies on a circle with center a

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In the xy-plane, point (r, s) lies on a circle with center a  [#permalink]

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New post 07 Jan 2014, 05:17
5
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A
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D
E

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The Official Guide For GMAT® Quantitative Review, 2ND Edition

In the xy-plane, point (r, s) lies on a circle with center at the origin. What is the value of r^2 + s^2?

(1) The circle has radius 2
(2) The point \((\sqrt{2}, \ -\sqrt{2})\) lies on the circle

Data Sufficiency
Question: 22
Category: Geometry Simple coordinate geometry
Page: 154
Difficulty: 600


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Re: In the xy-plane, point (r, s) lies on a circle with center a  [#permalink]

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New post 07 Jan 2014, 05:17
5
17
SOLUTION

THEORY:
In an xy-plane, the circle with center (a, b) and radius r is the set of all points (x, y) such that:
\((x-a)^2+(y-b)^2=r^2\)

Image

This equation of the circle follows from the Pythagorean theorem applied to any point on the circle: as shown in the diagram above, the radius is the hypotenuse of a right-angled triangle whose other sides are of length x-a and y-b.

If the circle is centered at the origin (0, 0), then the equation simplifies to: \(x^2+y^2=r^2\).

For more on this subject check Coordinate Geometry chapter of Math Book: math-coordinate-geometry-87652.html

BACK TO THE ORIGINAL QUESTION:
In the xy-plane, point (r, s) lies on a circle with center at the origin. What is the value of \(r^2 + s^2\)?

Now, as \(x^2+y^2=r^2\) then the question asks about the value of radius^2.

(1) The circle has radius 2 --> radius^2=4. Sufficient.

(2) The point \((\sqrt{2}, \ -\sqrt{2})\) lies on the circle --> substitute x and y coordinates of a point in \(x^2+y^2=r^2\) --> \(2+2=4=r^2\). Sufficient.

Answer: D.
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Re: In the xy-plane, point (r, s) lies on a circle with center a  [#permalink]

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New post 07 Jan 2014, 22:33
If a circle, lying on a xy-plane, has its center at the origin, the equation is x^2+y^2=R^2, where x & y are points on the circle and R is the radius of the circle.

Since x & y from the equation x^2+y^2=R^2 is similar to r & s in the question, we can rewrite as r^2+s^2=R^2.

Statement (1) gives us the value of radius, R; Therefore, r^2+s^2=4; Sufficient.
Statement (2) gives us the value of a point on the circle
=> sub x & y values of the point in r^2+s^2=R^22: 2+2 = 4;
r^2 = 4; Sufficient.

Hence, Answer is (D).
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Re: In the xy-plane, point (r, s) lies on a circle with center a  [#permalink]

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New post 08 Jan 2014, 09:04
1
1
Bunuel wrote:
The Official Guide For GMAT® Quantitative Review, 2ND Edition

In the xy-plane, point (r, s) lies on a circle with center at the origin. What is the value of r^2 + s^2?

(1) The circle has radius 2
(2) The point \((\sqrt{2}, \ -\sqrt{2})\) lies on the circle



The formula of circle is given by (x-a)^2 + (y-b)^2 = r^2 where circle is centered as (a,b) and r = radius of the circle.

So if the circle is centered at origin, then the equation reduces to x^2 + y^2 = r^2.

Statement 1) (r,s) lies on the circle and hence r^2 + s^2 should be equal to the radius of the circle and the statement provides that very fact. Hence Sufficient.
Statement 2) \((\sqrt{2}, \ -\sqrt{2})\) lies on the circle. Then the radius can be calculated as root(root(2)^2) + root(2)^2) = 2 and as the radius is same from any point on the circle. Hence Sufficient.

Option D)
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Re: In the xy-plane, point (r, s) lies on a circle with center a  [#permalink]

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New post 11 Jan 2014, 07:14
SOLUTION

THEORY:
In an xy-plane, the circle with center (a, b) and radius r is the set of all points (x, y) such that:
\((x-a)^2+(y-b)^2=r^2\)

Image

This equation of the circle follows from the Pythagorean theorem applied to any point on the circle: as shown in the diagram above, the radius is the hypotenuse of a right-angled triangle whose other sides are of length x-a and y-b.

If the circle is centered at the origin (0, 0), then the equation simplifies to: \(x^2+y^2=r^2\).

For more on this subject check Coordinate Geometry chapter of Math Book: math-coordinate-geometry-87652.html

BACK TO THE ORIGINAL QUESTION:
In the xy-plane, point (r, s) lies on a circle with center at the origin. What is the value of \(r^2 + s^2\)?

Now, as \(x^2+y^2=r^2\) then the question asks about the value of radius^2.

(1) The circle has radius 2 --> radius^2=4. Sufficient.

(2) The point \((\sqrt{2}, \ -\sqrt{2})\) lies on the circle --> substitute x and y coordinates of a point in \(x^2+y^2=r^2\) --> \(2+2=4=r^2\). Sufficient.

Answer: D.
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

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: In the xy-plane, point (r, s) lies on a circle with center a  [#permalink]

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New post 09 Oct 2016, 02:09
So laying on circle means on the edge of the circle? could not mean inside the circle? Someone please clarify.
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Re: In the xy-plane, point (r, s) lies on a circle with center a  [#permalink]

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New post 09 Oct 2016, 02:23
1
TheLordCommander wrote:
So laying on circle means on the edge of the circle? could not mean inside the circle? Someone please clarify.


Yes, "on a circle " means on the edge only. Had the point been inside the circle, it would have been clearly mentioned.
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Re: In the xy-plane, point (r, s) lies on a circle with center a  [#permalink]

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