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Annular ring

A chord of fixed length [4 units] is drawn on a circular GREEN disc. A circular hole that is tangent to chord is then drilled through the disc - leaving a GREEN ring. Why does the area of the ring not change as you vary the size of the disc and the hole?
Do you regard this applet as the basis for a student task in algebra? geometry? both? neither? Would you give this task to middle school students? Why or why not?