# Thread: proportional relationship between variables

1. ## proportional relationship between variables

Does a proportional relationship have to start at (0,0)? I know that if they are continuous then a proportional relationship contains the point zero. What about a discrete relationship? Can a discrete relationship be proportional?

2. It's a little hard to understand what you are talking about but apparently you are asking if a the graph of a "proportional relationship" (y= ax for direct proportion) contains the point (0, 0). The answer is yes, if "0" is in the domain but that is not always true.

I am assuming here that you mean "direct" proportion. The graph of an "inverse" proportion (y= a/x) never contains (0, 0).

3. Originally Posted by HallsofIvy
It's a little hard to understand what you are talking about but apparently you are asking if a the graph of a "proportional relationship" (y= ax for direct proportion) contains the point (0, 0). The answer is yes, if "0" is in the domain but that is not always true.

I am assuming here that you mean "direct" proportion. The graph of an "inverse" proportion (y= a/x) never contains (0, 0).
ok so, if I have a beaker and put 200ml water in it and place a marble in the beaker to determine its volume, the realationship between total volume is a direct proportion? because it causes the water to rise by a constant "K" each time, and the graph will start at (0,200).

4. Yes, the graph is linear. I would call that a "linear" relation, not a "proportional" relation because, while x= 0 (no marble) is in the domain, (0, 0) is not on the graph.