Understanding Reflecton in a line

Understand the Reflection in a line. Click on the link to Watch the VIDEO explanation:
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Reflection in a Line
 
Reflection of a Point in a line
We are given a line XY and a point P outside it
we are required to obtain the reflection of P in XY
The steps of construction are as follows
Construct a perpendicular from P to XY which meet XY at M.
Produce PM to P dash such that PM is equal to PM dash
Then, P dash is the image of P in XY
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Reflection of a line in a line
we are given the lines XY and AB.
We are required to construct the reflection of AB in XY
Proceed as follow
From A, construct AM is perpendicular XY and produce it.
Obtain point A dash i.e., the reflection of A in XY on the other side of XY, so that AM is equal to A dash  M
Similarly, from B, construct BN perpendicular to XY and produce it.
Obtain point B dash i.e., the reflection of B in XY on the other side of XY, so that BN is equal to B dash N
Join A dash B dash
Therefore, A dash B dash is the reflection of AB in XY.
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Reflection of a Point Through a Point
Given Points A and M
We are required to construct the reflection of point A through point M.