# 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.