There are two points A(x1, y1) and B(x2, y2). The coordinates of their midpoint P are ((x1+x2)/2, (y1+y2)/2)

In addition: If the symmetric point of any point (x, y) about (a, b) is (2a-x, 2b-y), then (2a-x, 2b-y) is also on this function. We have f(2a-x)= 2b-y. By moving the terms, we have y=2b- f(2a-x)