Anonymous asked in Science & MathematicsMathematics · 1 decade ago

What is the perimeter of a semi-circle of radius 2" ?


By the definition of a circle, only the points on the rim are the points of the

circle & those inside or outside the rim are not, why diameter is taken into account? Also,what is the perimeter of half of a rectangle of 2"x1". If we draw lines inside to bisect it, there are more

than 1 bisection and there will be more than 1 answer to the question. Please

explain, thanks.

Update 2:

Thank all concerning the problem. I have

already figured out the solution myself. It is that a semi-circle is simply an arc,it should not have "perimeter". But if is cut

out and bent into a circle of radius 1" like

a piece pf string, then its circumference or perimeter is exactly 2pi. So the answer is 2pi.

8 Answers

  • feanor
    Lv 7
    1 decade ago
    Favorite Answer

    pi r + 2 r = r (2 + pi)

    Perimeter of circle = pi d = 2 pi r

    But you have half a circle so it's "pi r"

    But don't forget about the straight edge = 2r


    Think of perimeter as the amount of material or the length of the "line" that you will need to draw the shape.

    If you draw just the half circle, it's simply a curve. Which you can think of as a shape with no area. Then it doesn't really have a perimeter because it doesn't have any area.

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  • Dodgey
    Lv 4
    1 decade ago

    The calculation for the perimiter of a circle = Pi X diameter, which for a whole circle with radius 2" would be 4 X Pi = 12.57".

    Now, you just want this for a semi-circle so cut that above number in half and you get 6.28" for the length of the semi-circular arc.

    However, the definition of Perimeter is the path that surrounds an area. Since the semi-circular arc does not fully surround the semi-circle you must add in the diameter (the straight line through the middle) as well as the arc to close the perimiter. Since the radius is 2", the diameter is 4".

    6.28" + 4" = 10.28"

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  • 4 years ago

    If I understand you correctly: The perimeter of the whole circle is 2*pi*r = 2*pi*25 cm The perimeter of the *semi*circle' arc is half that- ie pi*25 cm

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  • Anonymous
    1 decade ago

    well in terms of Pi or what? you need to specify more. or is it the perimeter of a semi circle when Pi is 3.14. Whatever it is this would be the formula

    2 x Pi x radius which is all divided by 2 then 4 is added because it is a half circle so u do the normal perimeter of a circle then add 4 because 4 is the diameter.

    So in terms of Pi it is 2Pi + 4 or if Pi is 3.14 it is 10.28

    Source(s): my mind
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  • 1 decade ago

    1/2( 2pi 2) the perimeter of a circle equals 2pi r but since this is a semi circle it is half the perimeter of a complete circle.

    So the answer is 2pi

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  • Pamela
    Lv 6
    1 decade ago

    A semi-circle's perimeter has two parts, the diameter of length 2r and the half circular arc of length πr.

    P = 2r + πr

    Fill in the value of r and you're done.

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  • vij
    Lv 6
    1 decade ago

    Perimeter of a semi circle = pi(r) + Diameter

    Perimeter of semi circle = 3.14(2inches) + 2(2inches)

    = 10.14 inches (Answer)

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  • 1 decade ago

    The formula for circumference is pi * d. Thus, it would be 2pi

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