TR(y) = yP(y) = ay by2,so
MR(y) = a 2by.These functions are shown in the following figure.
TR(y) = yP(y) = a,so MR(y) = 0. That is, the revenue obtained from selling an extra unit is zero: the price goes down enough to cancel out any gain.
TR(y) = yP(y) = a/y,so
MR(y) = a/y2.That is, the revenue obtained from selling an extra unit is negative: the price goes down enough to cause an increase in volume to lead to a decrease in revenue.