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Homework Help: Segment of a circle (Exact form answer)

  1. Sep 17, 2014 #1
    1. The problem statement, all variables and given/known data

    Find the circle segment area that has the boundaries of line segment AB and the minor arc ACB.

    Give the area in an exact form in terms of surds and Pi. (see attachments for annotated picture & original question).

    2. Relevant equations

    Equation 1: Area of a segment = Sector area - Triangle area

    Equation 2: Sector area = Central angle/360 * pi * diameter

    Equation 3: Triangle area = 0.5 * base * height

    3. The attempt at a solution

    Area of sector = 120/360 * pi * 4
    Simplified = 1/3 * pi/1 * 4/1 = 4/3 * pi [Units squared]

    Area of one of the triangle = 0.5 * 1√3 * 1
    Combined triangle area = 0.5 √3 x 2 = 1√3

    Segment area = (4 * pi/3 - √3) [Units squared]
    Approximated answer = 2.4567

    Attached Files:

  2. jcsd
  3. Sep 17, 2014 #2


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    Staff: Mentor

    Hi Matt. I see you are new here .... http://imageshack.com/a/img515/4884/welcomesign.gif [Broken]

    Your answer is right. Your working is very well presented.

    I would not include the real approximation, since it's the exact form that is required.

    Can you see a way to check your answer yourself?
    Last edited by a moderator: May 6, 2017
  4. Sep 17, 2014 #3
    Hello Nascent Oxygen,

    Thank you for your feedback, I appreciate it.

    The only methods I can think to check if the answer is correct is to use one of the following equations. I know those equation give the same approximate answers. However i'm not sure if that is a reliable method of checking if the answer is correct.

    Equation 1 (Area of a circle segment using height and radius)
    Radius [squared] * cos -1 (radius - height / radius) - (radius - height) √2*r*h -h2

    Equation 2 (Area of circle segment given the central angle)
    Radius [Squared] * (pi/180) * central angle - sin (central angle)
    Last edited by a moderator: May 6, 2017
  5. Sep 17, 2014 #4
    Thank you very much for your warm welcome. It's good to be on here.
  6. Sep 17, 2014 #5


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    Staff: Mentor

    I did have in mind just reversing the process to arrive at the area of the full sector, as a check of your arithmetic.

    But, you're right, you could compare your answer with a published formula, as you quoted.

    http://d2gbom735ivs5c.cloudfront.net/m/geometry/images/circle-segment-area.gif [Broken]
    Last edited by a moderator: May 6, 2017
  7. Sep 18, 2014 #6
    Just to check I understand correctly. I would calculate the approximate area of the segment area and the approximate area of the triangle and add them together.

    Segment area (2.4567) + Triangle area (1.7320) = Sector area (4.1887)

    4 * pi / 3 + √3 = 120/360 * pi * r [squared]
  8. Sep 18, 2014 #7


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    Staff: Mentor

    Any arithmetic check will pick up blunders, giving you a chance to rectify mistakes before they cost you marks.

    Now, 3 * 4.1887 = 12.5661

    compare with the area of a circle of radius 2,
    and there is agreement.
  9. Sep 23, 2014 #8
    Thank you Nascent Oxygen,

    That has really cleared things up for me. I have two questions for you, if you don't mind answering them. The first question refers to the answer having to be exact form. If I was to use the equation (below) that uses the circle's height and radius to calculate the segment area, would that be acceptable?

    Equation (Area of a circle segment using height and radius)
    Radius [squared] * cos -1 (radius - height / radius) - (radius - height) √2*r*h -h2
    Simplified form (Exact form) = 4 cos -1 (0.5) - √3

    The second question refers to that equation itself. I understand slightly how it works, however i'm slightly confused on exactly how it calculates the segment area.

    r2 cos -1 (radius - height / radius) = 4 pi / 3
    I don't understand how this works.
  10. Sep 23, 2014 #9


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    Staff: Mentor

    Can be simplified further, you probably know cos-1 ½ = π/3.

    Where you are given radius and height, draw also that radius onto which you can mark the height, then find the lengths of all 3 sides of the right-angled triangle formed at the circle's centre.

    Denote the angle at the centre as βẞand express βẞin terms of r and h.

    The double angle formula will around this time come in handy, because θ = 2βẞ
    Last edited: Sep 23, 2014
  11. Sep 24, 2014 #10
    I understand that cos-1 = pi/3. However i'm very confused regarding the rest. I understand using the radius to work out the hypotenuse of the triangle (Formed at the circle's centre) and the segment height is the same as the triangle's base.

    I'm unsure about denoting the angle at the centre (120 degrees) as βẞ expressed in terms of r and h.
  12. Sep 24, 2014 #11


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    Staff: Mentor

    βẞis 60o here

    βẞ= cos-1 .....
    where ..... is some expression in r and h
    Last edited: Sep 24, 2014
  13. Feb 10, 2015 #12
    Hello Nascent Oxygen, Totally forgot to thank you for all your help. So here it is, thank you very much. :)
  14. Feb 10, 2015 #13


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    Staff: Mentor

    Appreciate your reply. Also reveals that along with the β character I've been copying a strange character apparently concealed on my browser by a backspace. :eek:
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