Page "Cartesian coordinate system" Paragraph 38

If ( x, y ) are the Cartesian coordinates of a point, then (− x, y ) are the coordinates of its reflection across the second coordinate axis ( the Y axis ), as if that line were a mirror.

Likewise, ( x, − y ) are the coordinates of its reflection across the first coordinate axis ( the X axis ).

In more generality, reflection across a line through the origin making an angle with the x-axis, is equivalent to replacing every point with coordinates ( x, y ) by the point with coordinates ( x < nowiki >'</ nowiki >, y < nowiki >'</ nowiki >), where