有偿悬赏两道AS Econ题目

100论坛币悬赏2道AS Econ题目，难度不大，但是有点困惑

需要大神们告诉为什么选某一个选项，而其他选项为什么错；如果能告知技巧和原理就更好，谢谢！

Q1) CS = 0.5*(30-P)*q, while q = 0.5(-p + 30).
Therefore, CS = 1/4 * (30-p)^2.
CS - CS' = 1/4 * (30-p)^2 - 1/4 * (30 - p')^2 = 1/4 * (30-p)^2 - 1/4 * (25 - p)^2 = 5/4 * (55 - 2p), a decreasing function of p.
Taking derivatives, dCS/dp = 1/2 * (p - 30) < 0 (As p rises, CS decrease).
Taking the second derivative, dCS^2/dp^2 = 1/2 = constant.
Therefore, as p rises, CS falls at a constant rate.
The key is A.

Q2) The key is A.
Consider picture A. For p > pE, find the quantity of supply and quantity of demand using y = p. There is a supply shortage as S < D. Gradually, price will go up and producer would increase to response to a higher price. However, at a higher price, the D is also larger, still exceeding S. The persistent supply shortage keeps price, supply, and demand up, in a spiral route. That is, they will never return to the equilibrium.
Case B is a demand rigidity. When p > pE, S will exceed D, that is, a supply surplus. Price will decrease, then surplus is decreased. This process will continue until p returns to pE.
Case C is just the standard model.
In case D, for p > pE, we have a supply surplus. Price will then decrease, resulting in a smaller supply surplus. Gradually, p will return to the original pE. 使用说明.doc (23 KB, 需要: 100 个论坛币)

2021-4-22 21:31:33 上传

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Zoeyeyeyeye 发表于 2021-4-2 21:54
Q1) CS = 0.5*(30-P)*q, while q = 0.5(-p + 30).
Therefore, CS = 1/4 * (30-p)^2.
CS - CS' = 1/4 * ( ...

谢谢大佬，很受用，悬赏怎样支付

Akongform 发表于 2021-4-21 17:52
谢谢大佬，很受用，悬赏怎样支付

额，我修改了答案，随便放了个附件上去，设置了悬赏金额，现在应该可以付啦~

Zoeyeyeyeye 发表于 2021-4-22 21:34
额，我修改了答案，随便放了个附件上去，设置了悬赏金额，现在应该可以付啦~

哈哈哈哈哈大佬查收