为什么说MP=AP时，AP最大？过程如何推导？

老师，请问Fixed charge coverage中的 分子的lease payment是作为lessee的OL还是lessor的OL，那分母的lease payment是作为essee的OL还是lessor的OL？麻烦老师解读一下。谢谢

A mutual fund manager wants to create a fund based on a high-grade corporate bond index. She first distinguishes between utility bonds and industrial bonds; she then, for each segment, defines maturity intervals of less than 5 years, 5 to 10 years, and greater than 10 years. For each segment and maturity level, she classifies the bonds as callable or noncallable. She then randomly selects bonds from each of the subpopulations she has created. For the manager’s sample, which of the following best describes the sampling approach? A Stratified random. B Simple random. C Systematic. 上一题  正确答案A 您的答案B 本题平均正确率：92% Sampling, time-series and cross-sectional data难度：一般 推荐：      答案解析 In stratified random sampling, one divides the population into subpopulations and randomly samples from within the subpopulations. 请问： 1.分层是一定要求每组中有固定比例吗？ 2.系统抽样是每组中第Kth个的那种吗？

kinked model中MR为什么会有缺口？

弹性为1时，revenue最大。为什么？

Which of the following characteristics of an investment study most likely indicates time-period bias? A The study is based on a short time-series. B Information not available on the test date is used. C A structural change occurred prior to the start of the study’s time series. 查看解析 上一题 下一题 正确答案A 您的答案A本题平均正确率：65% Biases难度：一般 推荐：      答案解析 A short time series is likely to give period-specific results that may not reflect a longer time period. 请问：C选项的如果不是发生在之前 结构性变化和时间性偏差有和关联，可否举例解释一下，便于理解哈？

The aggregate demand curve是什么？经济学里有这个概念吗？

net tax=？

问题2：同样面积（这题的90%）的情况下，一般是不是t分布的对应的t值要比z分布对应的z值要大一点？有这个规律吗？为啥?

套利收益就是变成别的币种比原币种直接投资多的收益是吗？视频没有讲套利收益，不太明白什么是套利收益，麻烦老师解答一下，谢谢