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

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In the xy-plane, point (r, s) lies on a circle with center [#permalink]  12 Apr 2005, 13:56
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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
[Reveal] Spoiler: OA

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Last edited by Bunuel on 11 Feb 2012, 05:32, edited 2 times in total.
Edited the question and added the OA
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Re: DS circle [#permalink]  12 Apr 2005, 15:26
saurya_s wrote:
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 (v2, -v2) lies on the circle.

(1) r^2 + s^2 is the square of the radius of the circle. Sufficient.

(2) This is of no consequence since for any circle centered at the origin, there would be a point (v2. -v2) would lie on the circle. Gives us no info about r^2 + s^2.

Therefore, (A).
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In the xy-plane, point (r, s) lies on a circle with center [#permalink]  11 Feb 2012, 05:30
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DeeptiM wrote:
OA is D...can anyone explain??

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

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 $$x^2+y^2=radius^2$$ 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 $$x^2+y^2=radius^2$$ --> $$2+2=4=r^2$$. Sufficient.

Hope it helps.
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Re: In the xy-plane, point (r, s) lies on a circle with center [#permalink]  02 Mar 2013, 21:08
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Thanks for the brilliant explanation. One thing I don't get the question is that, the point (r,s) could be anywhere in the circle, not only on its circumference. Why does it refer only to a point on the circumference? Thanks!
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Re: In the xy-plane, point (r, s) lies on a circle with center [#permalink]  02 Mar 2013, 23:02
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ryusei1989 wrote:
Thanks for the brilliant explanation. One thing I don't get the question is that, the point (r,s) could be anywhere in the circle, not only on its circumference. Why does it refer only to a point on the circumference? Thanks!

It is the language.
On the circle = On the circumference.
In/Inside/Within the circle = Points enclosed by the circumference
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Re: In the xy-plane, point (r, s) lies on a circle with center [#permalink]  05 Jul 2014, 01:53
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In the xy-plane, point (r, s) lies on a circle with center [#permalink]  09 Jul 2014, 06:28
ryusei1989 wrote:
One thing I don't get the question is that, the point (r,s) could be anywhere in the circle, not only on its circumference.

Exactly.
In my opinion it's just poorly formulated as I've seen this exact questioning angle being used as a trap.
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Re: In the xy-plane, point (r, s) lies on a circle with center [#permalink]  09 Jul 2014, 06:30
Expert's post
mnlsrv wrote:
ryusei1989 wrote:
One thing I don't get the question is that, the point (r,s) could be anywhere in the circle, not only on its circumference.

Exactly.
In my opinion it's just poorly formulated as I've seen this exact questioning angle being used as a trap.

This doubt is addressed here: in-the-xy-plane-point-r-s-lies-on-a-circle-with-center-15566.html#p1191069

Point (r, s) lies ON a circle, means it's on the circumference.
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Re: In the xy-plane, point (r, s) lies on a circle with center [#permalink]  09 Jul 2014, 23:52
It is very interesting and difficult question. Thanks for all the solution of this...
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In the xy-plane, point (r, s) lies on a circle with center [#permalink]  09 Jul 2015, 01:18
Hi,

i don't understand from Bunuels solution how we come up with the value of S in statement 1 in order to answer what r^2 + s^2 is?

r^2 equals 4, that is all clear, but s should be y-value, how do we get that?
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Re: In the xy-plane, point (r, s) lies on a circle with center [#permalink]  09 Jul 2015, 02:43
Expert's post
noTh1ng wrote:
Hi,

i don't understand from Bunuels solution how we come up with the value of S in statement 1 in order to answer what r^2 + s^2 is?

r^2 equals 4, that is all clear, but s should be y-value, how do we get that?

$$r^2+s^2=radius^2$$. (1) says that radius = 2, thus $$r^2+s^2=2^2$$.
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Re: In the xy-plane, point (r, s) lies on a circle with center [#permalink]  09 Jul 2015, 02:52
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noTh1ng wrote:
Hi,

i don't understand from Bunuels solution how we come up with the value of S in statement 1 in order to answer what r^2 + s^2 is?

r^2 equals 4, that is all clear, but s should be y-value, how do we get that?

Hi noTh1ng,

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 (2√, −2√) lies on the circle

You seem to have misunderstood a little here.

The equation of Circle is given by $$x^2 + y^2 = Radius^2$$

Given : (r,s) lie on the circle
i.e. (r,s) will satisfy the equation of Circle
i.e. $$r^2 + s^2 = Radius^2$$

Question : Find the value of $$r^2 + s^2$$? but since $$r^2 + s^2 = Radius^2$$ therefore, the question becomes

Question : Find the value of $$Radius^2$$?

Statement 1: The circle has radius 2
i.e. $$r^2 + s^2 = Radius^2 = 2^2 = 4$$
SUFFICIENT

Statement 2: The point (√2, −√2) lies on the circle
i.e. (√2, −√2) will satisfy the equation of circle
i.e. (√2)^2 + (−√2)^2 = Radius^2
i.e. Radius = 4
hence, $$r^2 + s^2 = Radius^2 = 2^2 = 4$$ Hence,
SUFFICIENT

I hope it helps!

Please Note: You have been confused r (X-co-ordinate) and r (Radius) as it seems from your question
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In the xy-plane, point (r, s) lies on a circle with center [#permalink]  07 Jan 2016, 11:01
I'm quoting Bunuel's correct explanation below with some follow up questions, since this question's wording did not seem precise to me:

1. The given information says that point (r,s) lies on the circle. For future reference, will "on" mean on the circumference of the circle? In other words, "on" can never refer to a point inside the circle. If the point (r,s) were some arbitrary point inside of the circle, the solution to this problem would be incorrect I believe.

2. Is the point (r,s) assumed to NOT be constant? It seems like the explanation for working out (2) depends upon the fact that (r,s) is non-constant and could be ANY point along the circumference of the circle. If (r,s) were in fact constant, then I don't believe (2) would provide any information in deducing what r^2+s^2 is.

EDIT: I missed the above clarification regarding the language for "on the circle". I have crossed that question of mine out. Sorry!

---------------------------------------------------------------------------------------------------------
Bunuel wrote:
DeeptiM wrote:
OA is D...can anyone explain??

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 $$x^2+y^2=radius^2$$ 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 $$x^2+y^2=radius^2$$ --> $$2+2=4=r^2$$. Sufficient.

Hope it helps.
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Re: In the xy-plane, point (r, s) lies on a circle with center [#permalink]  07 Jan 2016, 11:44
lillylw wrote:
I'm quoting Bunuel's correct explanation below with some follow up questions, since this question's wording did not seem precise to me:

1. The given information says that point (r,s) lies on the circle. For future reference, will "on" mean on the circumference of the circle? In other words, "on" can never refer to a point inside the circle. If the point (r,s) were some arbitrary point inside of the circle, the solution to this problem would be incorrect I believe.

2. Is the point (r,s) assumed to NOT be constant? It seems like the explanation for working out (2) depends upon the fact that (r,s) is non-constant and could be ANY point along the circumference of the circle. If (r,s) were in fact constant, then I don't believe (2) would provide any information in deducing what r^2+s^2 is.

EDIT: I missed the above clarification regarding the language for "on the circle". I have crossed that question of mine out. Sorry!

For analysing statement 2, it does not matter whether (r,s) is constant.

Equation of a circle with center (a,b) and radius R is $$(x-a)^2+(y-b)^2=R^2$$ , now as the given circle is centered at (0,0) ---> a=b=0 ---> the equation of the circle thus becomes $$x^2+y^2 = R^2$$

As (r,s) lies on the circle ---> you can substitute r for x and s for y in the equation of the circle to get, $$r^2+s^2=R^2$$

Per statement 2, [$$\sqrt{2} , -\sqrt{2}$$] lies on the circle ---> from equation of the circle $$x^2+y^2 = R^2$$ ---> $$(\sqrt{2})^2+(-\sqrt{2})^2=R^2$$

But as mentioned above, $$r^2+s^2=R^2$$ ---> $$r^2+s^2 =(\sqrt{2})^2+(-\sqrt{2})^2 = 4$$ = a unique value.

Thus, it does not matter what particular values r and s take. Whatever set of (r,s) values we will get, will always satisfy the equation $$r^2+s^2=4$$. Some of the sets can be

$$(\sqrt{2}), -\sqrt{2})$$ or
$$(1, -\sqrt{3})$$ or
$$(1, \sqrt{3})$$ or
$$(-\sqrt{3},1)$$ or
$$(\sqrt{3}, 1)$$ etc

Hope this helps.
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In the xy-plane, point (r, s) lies on a circle with center [#permalink]  07 Jan 2016, 11:46
Quote:
For analysing statement 2, it does not matter whether (r,s) is constant.

Equation of a circle with center (a,b) and radius R is $$(x-a)^2+(y-b)^2=R^2$$ , now as the given circle is centered at (0,0) ---> a=b=0 ---> the equation of the circle thus becomes $$x^2+y^2 = R^2$$

As (r,s) lies on the circle ---> you can substitute r for x and s for y in the equation of the circle to get, $$r^2+s^2=R^2$$

Per statement 2, [$$\sqrt{2} , -\sqrt{2}$$] lies on the circle ---> from equation of the circle $$x^2+y^2 = R^2$$ ---> $$(\sqrt{2})^2+(-\sqrt{2})^2=R^2$$

But as mentioned above, $$r^2+s^2=R^2$$ ---> $$r^2+s^2 =(\sqrt{2})^2+(-\sqrt{2})^2 = 4$$ = a unique value.

Thus, it does not matter what particular values r and s take. Whatever set of (r,s) values we will get, will always satisfy the equation $$r^2+s^2=4$$. Some of the sets can be

$$(\sqrt{2}), -\sqrt{2})$$ or
$$(1, -\sqrt{3})$$ or
$$(1, \sqrt{3})$$ or
$$(-\sqrt{3},1)$$ or
$$(\sqrt{3}, 1)$$ etc

Hope this helps.

Perfect thank you! I understand this now!
In the xy-plane, point (r, s) lies on a circle with center   [#permalink] 07 Jan 2016, 11:46
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