这两天学习垄断市场时候遇到的一道随堂计算原题如下
Suppose you are the manager of Alpha Enterprises - a firm
that holds a patent that makes it the exclusive manufacturer
of bubble memory chips. Based on the estimates provided
by a consultant, you know that the relevant demand and cost
functions for bubble memory chips are
Q=25-0.5P    C=50+2Q
其中一问是What is the firm's marginal revenue when producing 4
units of output?
我当时的解法是按照求导的方法,具体如下:
P=50-2Q   then TR=P*Q=50Q-2Q^2
hence MR=D(TR/Q)=50-4Q
since Q=4   so MR=34
但是后来我发现按照另一个公式计算就是课本上的原始定义
MR=extra revenue from making and selling 1 more unit
     =[extra renvenue from selling last unit alone]-[revenus lost selling existing units at a lower price]
如此代入数值计算的话  就是 42-(44-42)*3=36

两个不同的结果  是错在哪里呢？