# MR Diagrams touching y axis

The following is from an exam question:

To my mind this is plain nonsense. If Q is zero, AR and MR are zero, not some point above P1.

My question: is there any significance in drawing this diagram with lines extended all the way to zero on the x-axis? Why not just start at q=1 not q=0?

From the Mankiw textbook we have the same approach, but this looks more deliberate, as cost lines don't claim to have values for q=0

In a sense it doesn't matter, but is it not just adding quite un-needed confusion? What am I missing?

The case of average revenue ($$AR$$) is a bit more complicated. Here it is more natural to think of the monetary amount as determined by the quantity sold ($$QS$$). We have:
$$AR=\dfrac{TR}{QS}$$
where $$TR$$ is total revenue. If $$QS=0$$ this reduces to zero divided by zero, which is not zero but undefined. Nevertheless, provided total revenue is a suitable function of quantity sold (a linear function for example as shown in the diagram in the question), average revenue at quantity zero can be defined as a limit:
$$AR(QS=0) = \lim_{QS \to 0}\dfrac{TR(QS)}{QS}$$