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Sample Standard Deviation Area of a Sector Simplifying Radicals with Variables Horizontal Line Test Factorial of a Number Graphing Slope, Undefined Slope, Flat Slope Area of a TriangleWorking out the area of a circle is usually relatively straightforward, provided you remember the
correct formula, which is as follows.

If you have a circle with radius ** r**,

then the area

Find the area of the following circle.

Area =

=

=

Find the area of the following circles.

Area =

A concrete race track is  10 meters wide, and has a diameter of  50 meters. Find the area of the concrete track.

Area of the concrete race track will be:

( AREA OF OUTER CIRCLE ) − ( AREA OF INNER CIRCLE )

Area of circle outer =>

From the information in the diagram, it can be established that the diameter of the inner circle is

As

Area of inner circle =>

Total area of concrete track is:

What is the area of a circle with circumference **80**cm?

This is another example that requires a little bit of extra work.

As long as you remember the formula for the circumference of a circle, then the area here can be
established.

Again answers are to  2 decimal places.

CIRCUMFERENCE =

( divide both sides by

AREA =

The total area of a circle with circumference **80**cm  is **509.10**cm^{2}.

Area of Quarter Circle

Sometimes when dealing with area of circle examples, it’s required to find the area of only a part of
a circle.

Usually this will be a half circle or a quarter circle

A half circle is also called a "semicircle".

The opening under a bridge creates a half circle, what is the area of the opening?

This example is asking to find the area of a semicircle.

The diameter is

So the first step is to work out what the area of the circle whole would be.

Area =

Now to obtain the area of the half circle or semicircle, we divide by  2 to find the area of the opening under the bridge.

As the area of the semicircle under the bridge is half what the area of the full circle with the same radius would be.

For area of circle examples involving a quarter circle, the general approach is the same, only the whole circle area will be divided by  4 instead of  2.

Area of circle whole would be

The area of the quarter circle is

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