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Area of an annulus
The number of square units it takes to completely fill an
annulus.
Try this Drag the orange dots to move and resize the circles. As the size of the circles
change, the area is recalculated.
Area formula
The area of the
annulus
is the area of the ringshaped space between the two circles that define it.
In the figure above it is the number of gray square units it takes to fill the annulus.
This area is the area of the entire disk, minus the area of the 'hole' in the middle. ( See Area of a Circle):

where:
R is the radius of the outer circle
H is the radius of the inner 'hole'
π is Pi, approximately 3.142

which simplifies a little to:
Try this
 In the figure above, click on "hide details"
 Drag the orange dots on the edge of the circles to make a randomsize annulus.
 Now try to estimate the area of the annulus just by looking at the squares inside it
 Calculate the area using the formula
When you done click "show details" to see how close you got.
Other circle topics
General
Equations of a circle
Angles in a circle
Arcs
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