What is the area of a circle?

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Presentation transcript:

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 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 = π2r Half of the circumference = πr Height = radius = r Height x Base = r π r = πr2 Area circle = πr2