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Reflection in the y-axis

Cartesian plane shown with two triangles ABC and A' B' C'.
Detailed description

The point \(A\) in the diagram above has been reflected in the \(y\)-axis. To reflect \(A\), a line has been drawn at right angles to the axis of reflection (the \(y\)-axis in this case) and a ruler or compass used to mark \(A^\prime\) the same distance from the axis of reflection, but on the other side. Similarly for \(B\) and \(C\).

Using coordinates we can describe the reflection of the vertices:

\(A(1,1) \rightarrow A^\prime (−1, 1); B(4, 2) \rightarrow B^\prime (−4, 2); C(2, 4) \rightarrow A^\prime (−2, 4)\).

In general, \(P(x, y) \rightarrow P^\prime (−x, y)\).