<u><big>Consumer surplus</big></u> equals buyers’ willingness to pay for a good minus the amount they actually pay, and it <u>measures the benefit buyers get from participating in a market.</u> Consumer surplus can be computed by finding the area below the demand curve and above the price.

上文涉及到的重要概念：Willingness to pay (支付意愿) is the maximum amount that a buyer will pay for a good. Consumer surplus (消費(fèi)者剩余) is the amount a buyer is willing to pay for a good minus the amount the buyer actually pays for it.
上文涉及到的數(shù)學(xué)公式：消費(fèi)者剩余 ＝ 支付意愿 － 實(shí)際價(jià)格.

<u><big>Producer surplus</big></u> equals the amount sellers receive for their goods minus their costs of production, and it <u>measures the benefit sellers get from participating in a market.</u> Producer surplus can be computed by finding the area below the price and above the supply curve.

上文涉及到的重要概念：Cost (代價(jià),生產(chǎn)成本,機(jī)會(huì)成本) is the value of everything a seller must give up to produce a good. Producer surplus (生產(chǎn)者剩余) is the amount a seller is paid for a good minus the seller’s cost of providing it.
上文涉及到的數(shù)學(xué)公式：生產(chǎn)者剩余 ＝ 實(shí)際價(jià)格 － 生產(chǎn)成本.

<u>An allocation of resources that maximizes[^fn] the sum</u> of consumer and producer surplus <u>is said to be <big>efficient</big>.</u> Policymakers are often concerned with the efficiency, as well as the equality, of economic outcomes.

上文涉及到的重要概念：Efficiency (效率) is the property of a resource allocation of maximizing the total surplus received by all members of society. Equality (公平) is the property of distributing economic prosperity uniformly among the members of society.
[^fn]:由此可見(jiàn)，作者 N. Gregory Mankiw (曼昆) 是個(gè)美國(guó)人。

The equilibrium of supply and demand maximizes the sum of consumer and producer surplus. That is, <u><big>the invisible hand</big></u>[^fn1] of the marketplace <u>leads buyers and sellers to allocate resources efficiently</u>.

上文涉及到的重要概念：Welfare economics (福利經(jīng)濟(jì)學(xué)) is the study of how the allocation of resources affects economic well-being. 關(guān)于「福利經(jīng)濟(jì)學(xué)」這一概念，我們需要注意的是，它關(guān)注的是 “how to affect” 的問(wèn)題（正如我們用 Italic 斜體 突出的字詞）。
[^fn1]:語(yǔ)出 Adam Smith, An Inquiry into the Nature and Causes of the Wealth of Nations (1776) —— 亞當(dāng)·斯密《國(guó)富論》。

<u>Markets do not allocate resources efficiently in the presence of <big>market failures</big></u> such as market power or externalities.

必須澄清的是：「市場(chǎng)失敗」不等于「自由市場(chǎng)一定是失敗的」?！甘袌?chǎng)失敗」說(shuō)的是，如果存在市場(chǎng)勢(shì)力和外部性，那么這種市場(chǎng)的自發(fā)均衡狀態(tài)不是最有效率的情況。當(dāng)我們從前提得出結(jié)論時(shí)，如果我們假設(shè)了這一前提是真的，那么按照合乎邏輯的步驟、最后得出的結(jié)論也一定是真的。問(wèn)題在于「前提可能是錯(cuò)的」，即使說(shuō)「前提可能不符合現(xiàn)實(shí)情況」。如果是這樣的，那么「假前提不能保證結(jié)論一定真」[^fn2]，即使說(shuō)「結(jié)論可能是真的，但這種真是偶然的」。從而，當(dāng)我們提及「市場(chǎng)失敗」時(shí)，我們是想表達(dá)「市場(chǎng)在某些情況下是無(wú)法有效配置資源的」這層意思。常見(jiàn)的2種情況，一個(gè)是「存在市場(chǎng)勢(shì)力」，一個(gè)是「存在外部性」。
[^fn2]:注意，我說(shuō)的是「不能保證」，我的意思是「從假前提得出的結(jié)論可真可假」。這里要注意的是，如果前提是真的，那么合乎邏輯的結(jié)論一定是真的；如果前提是假的，那么合乎邏輯的結(jié)論可真可假，即使說(shuō)，它「真假不定」。直白地說(shuō)，「真假不定」的句子對(duì)我們的生活沒(méi)有任何指導(dǎo)意義 —— 它是無(wú)意義的信息。