Arthur Lee

Describe the position of a point in the coordinate plane when that point is moving steadily on a line.

In this example, C is cyclone moving from A to B in 1 day. Let d be the number of days (which can be a fraction). Position of C can be described in terms of d and the coordinates of A and B. (In the HK DSE curriculum, this corresponds to the formula for internal point of division. More generally, this can be a vector representation of straight line.)