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Circumference of Circles

(See for anothernew version.) Drag the green point to appropriate position to measure the diameter of the circle. Drag the red point P to straighten the circumference. Divide the scale of the ruler to measure its length to two decimal places. Use the Zoom In, Zoom Out and Move tools if necessary. Calculate the values of c/d. What do you discover?
Anthony OR. GeoGebra Institute of Hong Kong.