A drop of water dripped into the newspaper. Why are traces of water round? Answer with mathematical knowledge.
According to the nature of the circle, the distance from the center of the circle to a point is equal, and then the connecting line rotates 360 degrees, then the swept area is a circle, because the diffusion speed is the same, that is to say, the speed * time = radius is also equal. Of course, the swept area is a circle.