Understand that a relationship between two variables, x and y, is proportional if it can be expressed in the form y/x=k or y=kx. The constant of proportionality k is given by k=y/x where y and x are two quantities that are directly proportional to each other. Once you know the constant of proportionality you can find an equation representing the directly proportional relationship between x and y, namely y=kx, with your specific k. If the relationship between two quantities is a proportional relationship, this relationship can be represented by the graph of a straight line through the origin with a slope equal to the unit rate. For each point (x, y) on the graph, ž is equal to k, where k is the unit rate. The point (1, k) is a point on the graph. Although the points fall on a line, the line does not pass through the origin, so the graph does not represent two quantities that are proportional to each other.
