$r(l)$ denote the reflection in a line $l$, and $u\in\mathbb{E}$ be any point in a euclideian space, we need to prove $r(l)^u=r(lu)$ where $lu$ is the image of $l$ under $u$
Hints: all points C in the plane equidistant from two given point A and B lie on a line $l$, called perpendicular bisector A and B, any isometry of the plane maps line to line.
thanks for helping. I have no clue to solve this