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解析幾何：線對稱
A 和 A' 的中點 M = (A+A') / 2
而且 M 會在直線 L 上
我們先設法向量為 u
可以寫出：
A' = A + t*u
所以 M = ( 2A + t*u ) / 2
再將 M 代入 L 的方程式中解 t
便可求得 A'