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# If a circle with center at the origin passes through the point (0,5),

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If a circle with center at the origin passes through the point (0,5),  [#permalink]

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16 Dec 2017, 02:08
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45% (medium)

Question Stats:

66% (00:58) correct 34% (00:55) wrong based on 87 sessions

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If a circle with center at the origin passes through the point (0,5), through which of the following points does it also pass?

(A) (2, √21)
(B) (4, 5)
(C) (5, √21)
(D) (5, 5)
(E) (5, 12)

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Re: If a circle with center at the origin passes through the point (0,5),  [#permalink]

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16 Dec 2017, 03:58
The answer is A I suppose, coz the distance from origin to (0,5) is 5 so any option that satisfies the distance as 5 could be the answer. All suggestions accepted if I m wrong in good humour
Cheers,
Karthik

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Re: If a circle with center at the origin passes through the point (0,5),  [#permalink]

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16 Dec 2017, 06:52
circle with center at the origin passes through the point (0,5) , this means radius = 5.
we need to find another point which is at a distance of radius(5) from centre (0,0).
Only option A satisfies this...
Distance of (2, root 21) from (0,0) = (4+21)^0.5 = 5.

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Re: If a circle with center at the origin passes through the point (0,5),  [#permalink]

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16 Dec 2017, 08:33
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Bunuel wrote:
If a circle with center at the origin passes through the point (0,5), through which of the following points does it also pass?

(A) (2, √21)
(B) (4, 5)
(C) (5, √21)
(D) (5, 5)
(E) (5, 12)

Given: a circle centered at the origin is equal to coordinates (0,0) and the circle that passes through the point (0,5) has a radius equal to 5.
Question: find the choice that has the same distance as the radius of the circle = 5.

To calculate the distance between the origin and a point: $$\sqrt{(x^2+y^2)}$$

Note: most choices contain a 5 and anything with a square root greater than 25 is larger than 5, so eliminate all choices that contain 5 = BCDE.

(A) (2, √21) = $$\sqrt{(2^2+√21^2)} = √25 = 5$$ this is the correct choice.

(B) (4, 5) = √(16+25) > 5
(C) (5, √21) = √(25+21) > 5
(D) (5, 5) = √(25+25) > 5
(E) (5, 12) = √(25+144) > 5
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If a circle with center at the origin passes through the point (0,5),  [#permalink]

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19 Dec 2017, 09:43
Bunuel wrote:
If a circle with center at the origin passes through the point (0,5), through which of the following points does it also pass?

(A) (2, √21)
(B) (4, 5)
(C) (5, √21)
(D) (5, 5)
(E) (5, 12)

The basic equation of a circle yields the answer quickly.

A circle with center at origin* has the basic equation (Pythagorean theorem, where r = h):
$$x^2 + y^2 = r^2$$

Radius, r = 5. From (0,5) and (0,0): (5-0) = 5

This circle's equation is
$$x^2 + y^2 = 5^2$$
$$x^2 + y^2 = 25$$

Any two points on the circle must satisfy the equation: Square each coordinate. They must now sum to 25.

Eliminate answers B, C, D, and E

All have 5 as one coordinate.$$5^2 = 25$$
The other coordinate would have to be 0 for the Pythagorean theorem to hold.

By POE, the answer is A (2, √21)
And $$(2^2 + √21^2) = (4 + 21) = 25$$

In case there is doubt:
$$x^2 + y^2 = 25$$

(B) (4, 5)
$$(4^2 + 5^2) = (16 + 25) = 41$$
(C) (5, √21)
$$(5^2 + √21^2) = (25 + 21) = 46$$
(D) (5, 5)
$$(5^2 + 5^2) = (25 + 25) = 50$$
(E) (5, 12)
$$(5^2 + 12^2) = (25 + 144) = 169$$

*If the circle is not centered at origin, that is, has center (h,k), the general equation for a circle is used
$$(x - h)^2 + (y - k)^2 = r^2$$

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