Trigonometry/Reflections in a line

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.