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What is the area of a circle?
Let’s see!(Also available at www.mrmartinweb.com
under Teaching Resources)
A circle with a diameter of 20 cm or a radius of 10cm on centimeter graph paper.
One way is to count the centimeter squares,estimating where there are partial squares.
Now we will find the formula.
• Circle divided into 16 segments.– How did I do that?
Color each half a different color
Cut out circle. Cut out segments.
Glue segments onto file folder, one color up, the other color down.
Time to discover!
• What shape is the figure?
• How do we find the area of that shape?
• What is the height?
• What is the base?
• What equation can you make?
Shape is a parallelogram.
Area of parallelogram is base x height
Base
Height
•What is the Base? •What is the Height?
The Formula for the Area of a Circle!
• Area parallelogram = Base x Height• Base is one half of the circumference
Circumference = π x Diameter = π2rHalf of the circumference = πr
• Height = radius = r• Height x Base = r π r = πr2
Area circle = πr2