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Draw a curve that is radius away

All points are the same distance from the center.

Put a pin in a board, put a loop of string around it, and insert a pencil into the loop. Keep the string stretched and draw the circle!

Try dragging the point to see how the radius and circumference change.

(See if you can keep a constant radius!)

TheRadiusis the distance from the center outwards.

TheDiametergoes straight across the circle, through the center.

TheCircumferenceis the distance once around the circle.

When we divide the circumference by the diameter we get 3.141592654…

So when the diameter is 1, the circumference is 3.141592654…

Distance walked = Circumference = 100m

Also note that the Diameter is twice the Radius:

The length of the words may help you remember:

The circle is a plane shape (two dimensional), so:

Circle: theset of all pointson aplanethat are a fixed distance from a center.

Thearea of a circleistimes the radius squared, which is written:

To help you remember think Pie Are Squared (even though pies are usually round):

A circle hasabout 80%of the area of a similar-width square.

The actual value is (/4) = 0.785398… = 78.5398…%

Because people have studied circles for thousands of years special names have come about.

Nobody wants to saythat line that starts at one side of the circle, goes through the center and ends on the other sidewhen they can just say Diameter.

So here are the most common special names:

A line that just touches the circle as it passes by is called aTangent.

A line that cuts the circle at two points is called aSecant.

A line segment that goes from one point to another on the circles circumference is called aChord.

If it passes through the center it is called aDiameter.

And a part of the circumference is called anArc.

There are two main slices of a circle.

The pizza slice is called aSector.

And the slice made by a chord is called aSegment.

The Quadrant and Semicircle are two special types of Sector:

Quarter of a circle is called aQuadrant.

Half a circle is called aSemicircle.

A circle has an inside and an outside (of course!). But it also has an on, because we could be right on the circle.

Example: A is outside the circle, B is inside the circle and C is on the circle.

A circle is a special case of anellipse.