Show $_p*$_l*$_p*$_l*$_p*$_l*$_p is a reflection in a line parallel to line l
note $_p is a reflection about the line p
We can suppose the line l is the x-axis. For the line p, we have two cases.
Case 1: line p is parallel to line l, we can suppose p has the equation y=c. Then for any point (a,b), we have $_l(a,b)=(a,-b) and $_p(a,b)=(a,2c-b). Thus we ...
This shows how to prove that a transformation is a reflection.