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# How do you know what the radius is if only the diameter is given in a question?

By Lee Mansfield on the 11th of June, 2012

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The radius of a circle is exactly half the diameter. For example, is the diameter of a circle is 4cm then the radius of the circle is 2cm.

By Stephen McQueen on the 11th of June, 2012

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Radius of a circle is the half of the diameter. If D is the diameter and r the radius then r=D/2.

By Adersh Antony on the 19th of January, 2013

## Solve problems involving perimeters and areas of circles

circle is a shape where every point on its edge is the same distance from the centre.

The distance from the centre to the edge of the circle is called the Radius.

The distance from one side of the circle to the other, going through the centre, is called theDiameter. This is also double the length of the Radius.

π (pronounced Pi) is a special number that is calculated by dividing the circumference of a circle by the diameter. No matter what size circle you have, this number always stays the same. In reality, this number is never ending, but we can approximate it to 3.14, but to make it easier for us, since this number never changes, we just call it Pi and write it π.

The circumference of a circle is just another way of saying the perimeter of a circle, the distance around the edge of a circle.

[incomplete]

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