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How do you find the area and perimeter of a circle and semi circle?

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circa1934 | Student, Undergraduate | eNotes Newbie

Posted June 2, 2009 at 6:43 AM via web

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How do you find the area and perimeter of a circle and semi circle?

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cburr | Middle School Teacher | (Level 2) Associate Educator

Posted June 2, 2009 at 8:58 AM (Answer #1)

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For both area and perimeter -- in a circle this is called the circumference -- you need to know the radius of the circle, which is HALF the distance across the circle through the midpoint.  Here are the formulas:

CIRCUMFERENCE: pi (3.14) x 2 x radius

AREA:                pi x (radius squared)

So, as an example, let's say we have a circle with a radius of 4.  Here are the calculations:

Circumference:  3.14 x 2 x 4 = 25.12

Area:               3.14 x 4^2 = 50.24

Of course, both the circumference and area of a semi-circle are one-half that of the whole circle.


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neela | High School Teacher | Valedictorian

Posted June 2, 2009 at 6:39 PM (Answer #2)

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Definition: The perimeter or the circumference of the circle is 2pir or 2pi times the radius of the cicrle. It is Pi timies the diameter or pid. Where radius is the constant distance from the centre of the circle to its circumference. Pi is a constant (not rational number) which is like 3.141592654.... For rough calculation, we use it near two .14decimal place value ,3/

Area is pir^2 or pi times the square of the radius. Also in termes of diameter, the area of the cicle is pi(d/2)^2 =pid^2/4.

Therefore, given the the radius or diameter you can find the

area of the cicle.

The semicle : Its perimeter = diameter+(1/2)the circumference =d+(1/2)pid=d(1+pi/2) =2r+pir=r(2+pi).


If a  just  picture of a cirle is given  without giving radius or diameter, and you are required to find its perimeter: Then follow like this:


Draw two non parallel chords AB, CD.  A,B,Cand D will be on the circumference of the cicle.

Draw perpendicular bisectors to AB and CD. Let them meet at O. Now O is the centre of the given circle. Then measure OA or OB or OX, where X is any point on the circumference. You will find thar OA=OB=OX = radius. Use the radius in the above given formula to find perimeter or area of circle or semicircle.

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