411 REFERENCE · Wikibooks
Trigonometry/Reflections in a line
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Given any point P and a line not passing through P, the reflection of P in that line is defined as follows:
Draw the perpendicular from P to the line; let it meet the line at O.
Extend the perpendicular to the other side of the line.
Let OQ equal OP in length.
Then Q is the reflection.
If the line passes through P, then P is its own reflection in that line.