A reflection over a line l moves every point
P of the plane to the point
P
′ on the perpendicular from P
to the line
l
and on the same distance from
l
.
If the line is the x -axis, a point
P = ( p
1 p
2
) moves to the point P
′ = (
p
1
,
−
p
2
)
.
The image of the triangle
A
B
C is the triangle A
′
B
′
C
′
with B
′ = (
9
,
−
5
)
