A proportional relation is the one that can be expressed using an equation with the following structure:
Where k is a constant of proportionality (it has to be equal for every value in the relationship).
1. Graph 1 shows a proportional relationship, this is because all values are ruled by the same constant of proportionality, we can observe it because the slope of the line is always the same.
2. Graph 2 does not show a proportional relationship, this is because although the graph has the same slope, it does not passes through the origin, which means that it does not follow the rule y=kx.
3. Graph 3 does not show a proportional relationship because it has more than one constant of proportionality, we can see it because the graph has two slopes.