Circle
A circle is the round shape, such as the letter o.
The diameter (meaning "all the way across"), or d, of the circle is equal to twice its radius. (d equals 2 times r, or d=2r). 'Diameter' means the straight line that goes from one side to the opposite and right through the point in the very middle.
The number pi, usually written using its symbol π, equal to about 22/7ths or 3.14159, is very useful for getting information about the circle:
- The circumference (c) (meaning "all the way around") of the circle is equal to π times the diameter. (c equals π times d, or c=πd)
- The area bounded by the circle (a) is equal to the radius multiplied by itself, an multiplied by π. (a equals π times (r times r), or a=πr^2)
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Circle