What is the area of a circle? Let’s see! (Also available at under Teaching Resources).

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