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### Circles A quick tutorial about circles to help you understand about calculating the area and circumference. Try the Online Circle Measurement Tool. If you know either the Diameter, Area or Circumference, the tool does the rest!

 Circle Lines in Circles AB = Diameter Must pass through the centre of the circle. The Diameter is equal to twice the radius. OC = Radius The radius is a line segment that begin from the centre and touches any point on the circle. ED = Chord The chord joins any two points on a circle. FG = Tangent A line with one point common to the circle. EHD = Arc Only the portion of the circle that is between two points on the circle. ADB = Semicircle Also considered to be an arc that is exactly half of the circle. OCB = Sector The area between two radii and the arc. COB = Central Angle Fromed by two radii.

The circumference of a circle is the actual length around the circle which is equal to 360°. Pi (p) is the number needed to compute the circumference of the circle.
p is equal to 3.14.
Pi is greek and has been around for over 2000 years!

In circles the AREA is equal to 3.14 (p) times the radius (r) to the power of 2.
Thus the formula looks like:
A= pr2.

In circles the circumference is 3.14 (p) times the Diameter.
Thus the formula looks like:
2pr or pd.