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# What is the measure of the radius of the circle that circums

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What is the measure of the radius of the circle that circums [#permalink]

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23 Jan 2010, 10:50
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79% (01:52) correct 21% (01:20) wrong based on 159 sessions

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What is the measure of the radius of the circle that circumscribes a triangle whose sides measure 9, 40 and 41?

(A) 6
(B) 4
(C) 24.5
(D) 20.5
(E) 12.5
[Reveal] Spoiler: OA

Last edited by Bunuel on 22 Nov 2013, 02:38, edited 1 time in total.
Renamed the topic, edited the question and added the OA.
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Re: Geometry - Triangle circumscribed in a circle [#permalink]

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23 Jan 2010, 11:18
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shuj00 wrote:
What is the measure of the radius of the circle that circumscribes a triangle whose sides measure 9, 40 and 41?
6
4
24.5
20.5
12.5
The correct choice is (D) and the correct answer is 20.5.

Triangle with sides 9, 40 and 41 is a right triangle

$$40^2+9^2 = 41^2$$

A right triangle incribed inside a circle - longest side of the triangle will be the diameter of the circle.

so dia of circle is 41 and so the redius is 20.5
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Re: Geometry - Triangle circumscribed in a circle [#permalink]

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23 Jan 2010, 11:24
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First, you can determine that you have a right triangle:

40^2 + 9^2 = 41^2

Because you have a right triangle, the 'circumcenter' of is on the hypotenuse. What this means is that the center of the circle lies on the hypotenuse of the right triangle. Because the 'circumcenter' is determined by the perpendicular bisectors of the midpoint of each side of the right triangle, we know that the circumcenter is at the midpoint of the hypotenuse.

Because we know that the hypotenuse of the right triangle goes through the center of the circle, the length of the hypotenuse is also the diameter of the circle. So, the radius is half the length of this side of the triangle.

Thus, 41 / 2 = 20.5.

There is a much better explanation on Wikipedia - search for 'Circumscribed circle' (I can't post links yet).
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Re: Geometry - Triangle circumscribed in a circle [#permalink]

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22 Nov 2013, 02:21
swatirpr wrote:
shuj00 wrote:
What is the measure of the radius of the circle that circumscribes a triangle whose sides measure 9, 40 and 41?
6
4
24.5
20.5
12.5
The correct choice is (D) and the correct answer is 20.5.

Triangle with sides 9, 40 and 41 is a right triangle

$$40^2+9^2 = 41^2$$

A right triangle incribed inside a circle - longest side of the triangle will be the diameter of the circle.

so dia of circle is 41 and so the redius is 20.5

Bunuel Boss any better approach I solved the same way as mentioned above.

I sit essential that longest side will be diameter?
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Re: Geometry - Triangle circumscribed in a circle [#permalink]

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22 Nov 2013, 02:44
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honchos wrote:
swatirpr wrote:
shuj00 wrote:
What is the measure of the radius of the circle that circumscribes a triangle whose sides measure 9, 40 and 41?
6
4
24.5
20.5
12.5
The correct choice is (D) and the correct answer is 20.5.

Triangle with sides 9, 40 and 41 is a right triangle

$$40^2+9^2 = 41^2$$

A right triangle incribed inside a circle - longest side of the triangle will be the diameter of the circle.

so dia of circle is 41 and so the redius is 20.5

Bunuel Boss any better approach I solved the same way as mentioned above.

I sit essential that longest side will be diameter?

What is the measure of the radius of the circle that circumscribes a triangle whose sides measure 9, 40 and 41?
(A) 6
(B) 4
(C) 24.5
(D) 20.5
(E) 12.5

First of all we can notice that a triangle whose sides measure 9, 40 and 41 is a right triangle because 9^2 + 40^2 = 41^2.

Next, 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 a circle is also the triangle’s side, then that triangle is a right triangle).

Thus the diameter of the circle is the hypotenuse of the triangle --> diameter = hypotenuse = 41 --> radius = 41/2 = 20.5.

Hope it's clear.
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Re: What is the measure of the radius of the circle that circums [#permalink]

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10 Mar 2014, 19:07

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Re: What is the measure of the radius of the circle that circums [#permalink]

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11 Jan 2016, 19:52
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Re: What is the measure of the radius of the circle that circums   [#permalink] 11 Jan 2016, 19:52
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