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Where are the Circumference, Radius, Diameter, Chord and Tangent on a Circle?

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In this hub I will explain to you the names of different parts of a circle.

1) The circumference is the distance around the circle (basically it’s the perimeter of the circle)

2) The diameter is the distance across the centre of a circle. The radius is the distance halfway across the centre of the circle (the radius is basically half of the diameter). For example, if the diameter of a circle is 24cm then the radius will be 12 cm (since ½ of 24 = 12).

The radius and diameter are important for working out the area and circumference of a circle. In order to work out the area of the circle you will need to square the radius of the circle, and multiply the answer by Pi. To work out the circumference of the circle you will need to multiply the diameter of the circle by Pi.


What is the area and circumference of a circle of radius 5cm?

So to find the area, square the radius to give 25 (5 multiplied by 5), and multiply the answer by Pi (Pi = 3.14) to give 78.5cm rounded to 1 decimal place.

Now to find the circumference, first double the radius of the circle as you will need the diameter of the circle. 5 doubled gives 10cm. Finally multiply the diameter by Pi to give 31.4cm to 1 decimal place.

The radius and diameter of a circle are also needed to work out other shapes related to circles. Here are some other useful links on these topics:

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Calculating the area of a semicircle.

Working out the perimeter of a semicircle.


3) A chord is a line that passes from through the inside of the circle, from one point on the circumference to another (as long as it doesn’t pass through the centre of the circle).

A chord and radius will bisect each other at right angles.


4) A tangent is a line that "just touches" the circumference of a circle from the outside.

You will use tangents when you look at circle theorems.

One of these circles theorems is that a tangent and radius make a right angle at the point of contact. The second theorem invloving tangents is the alternate segment theorem.

Also in advanced calculus, you make be asked to work out the equation of a tangent at a point on a curve.

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