Lecture
There is an O - a fixed point and a point A - an arbitrary point. Draw a line through the points AO. Let us delay OA `equal to OA from point O, so that OA and OA` are additional. Then the point A` is called the symmetric point A with respect to the point O.
The transformation of a figure F into a figure F`, in which each of its point A passes into a point A`, symmetric with respect to this point O, is called a symmetry transformation with respect to the point O. Then the figures F and F` are called symmetrical with respect to the point O.
If the symmetry transformation translates a figure into itself, then such a figure is called centrally symmetric. A parallelogram is a centrally symmetric figure.
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