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圓內一點到根軸的距離
圖形設定：
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[*]圓 A 為定圓，C 為圓上動點，D 為圓內定點。
[*]圖中紅色虛圓以 C 為圓心、CD 為半徑。
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幾何性質：
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[*]DG 為 D 到兩圓根軸的距離，其為定值。
[*]定值為[math]\frac{d^2-r^2}{2r}[/math]，其中 d 為 AD 距離，r 為圓 A 半徑。
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