第六章，关于三级价格歧视的例题
已知使用三阶价格定价中的群体定价，条件如下。
如何求需求曲线和消费者剩余？

The price and maximum
minutes per month for each plan are: Plan 1:
500 minutes for $50; Plan 2: 750 minutes for
$62.50; and Plan 3: 1000minutes for $75.00.
Assuming that there are equal numbers of
consumers in each group and that the value of
a marginal minute for each group declines at
the rate of $0.0004 perminute used,work out
the demand curves consistent with this pric-
ing. What surplus will each consumer group
enjoy