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

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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.

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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