At the exact midpoint of the demand curve, marginal revenue equals zero.
This is also the point where total revenue reaches its maximum value. In Figure 3.5, total revenue is at a maximum of $36 at a quantity of 6 units of output and a price of $6.
And as we established, demand is unit elastic at this price. Any small change in price at this point will have no impact on total revenue. Table 3.5 summarizes all of these relationships for a linear downward sloping demand curve.