All my students,
You would find the formulas for the area of a square or a rectangle easily. What about the area of a circle? By using this dynamic software, you will have a chance to see where the formula for the area of the circle comes from. Let's try.

When you increase in the number of dissected parts and then rearrange these parts, what would you observe about the shape?
According to first question, can you refer the formula for the area of this shape?
Where does the formula for the area of a circle come from?