因为回转体在特殊情况下投影为圆，此类问题就转化为求圆的半径、圆心及切线了。
On special conditions, their projections are circular, these problems turn into the solutions of the circle radius center and tangent.
因为回转体在特殊情况下投影为圆，此类问题就转化为求圆的半径、圆心及切线了。
On special conditions, their projections are circular, these problems turn into the solutions of the circle radius center and tangent.
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