The Book of Lemmas: Proposition 13
Let AB be the diameter of a circle and let it intersect any chord CD that crosses AB but is not a diameter at point E. If points F and G are located on CD such that AF and BG are drawn perpendicular to CD, then CG is equal to DF.
Click on the plot below to add chord CD to the diagram, ensuring that it crosses AB, the diameter of the circle. Then, click "Add segments AF and BG" to add these segments to the diagram and compare the lengths of CG and DF.
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