In this lesson, students see that the area of a circle can be found by multiplying . They look at informal dissection arguments to derive that relationship.

First, students cut and rearrange a circle into a shape that approximates a parallelogram (MP8). The next activity is optional because it shows a different way to cut and rearrange a circle into a shape that approximates a triangle. In each case, the area of the polygon is equal to . Using algebraic reasoning, students construct and critique arguments that this is equivalent to or (MP3). The little “2” up in the air is pronounced squared and means that the value of r is multiplied by itself. Finally, students apply the formula to solve problems.