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please explain gmat prep question

This topic has 1 expert reply and 2 member replies
yvonne12 Master | Next Rank: 500 Posts Default Avatar
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please explain gmat prep question

Post Sun Mar 04, 2007 12:27 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    an equilateral triangle is inscribed in a circle, if the length of arc ABC is 24, what is the approximate diamter of the circle?

    ans.11

    please explain

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    jayhawk2001 Community Manager
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    Post Sun Mar 04, 2007 6:14 pm
    Arc ABC covers 120 + 120 = 240 degrees

    240 / 360 * perimeter = 24
    = 2 / 3 * (pi * D) = 24

    So, D = 24 * 3 / (2 * pi) which yields 36 / pi which is approx 11

    yvonne12 Master | Next Rank: 500 Posts Default Avatar
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    Post Mon Mar 05, 2007 5:07 pm
    I appreciate the time you've taken, could you please tell me how you came to 240.

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    Stacey Koprince GMAT Instructor
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    Post Sun Mar 11, 2007 10:19 pm
    It's a bit tough to explain the angles without a diagram. Open up this link to follow along; focus on the 2nd circle on the page:
    http://mathworld.wolfram.com/CentralAngle.html

    Draw a circle with the inscribed triangle and label your three vertices A, B, and C. Also label arc ABC.

    Each angle of the triangle is 60 degrees (since it's equilateral). These angles are called "inscribed angles" because they are on the perimeter of the circle itself. These are the equivalent of the red angle shown in the link I pasted above.

    Any inscribed angle is exactly half of its corresponding central angle (which is the blue angle shown in the link I pasted above).

    We use the central angle to figure out what portion of a circle we are talking about. Since the arc we want, ABC, covers inscribed angle A (60 degrees) and inscribed angle C (also 60 degrees), the corresponding central angles are double, or 120 and 120. 120 + 120 = 240.

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