Drag the green point to appropriate position to measure the diameter of the circle. What is its diameter?[br][br]Drag the red point P to straighten the circumference. Divide the scale of the ruler to measure its length to [b]two decimal places[/b]. Use the Zoom In, Zoom Out and Move tools if necessary.[br][br]Change the diameter of the circle to 2 cm, 3cm, or any other diameters. Calculate the value of:[br]circumference ÷ diameter.[br]What do you observe?

Anthony Or. GeoGebra Institute of Hong Kong.[br]orchiming@gmail.com