shu ru bu liao zhong wen

The first sentence indicates that a firm's value P(AB) cannot be bigger than P(A)+P(B), because an investor can easily buy A and B separately and form a portfolio by herself, without paying extra money to the firm. So a firm can diversify but the diversification does not increase its value.

The second sentence says that the firm's value does not decrease if it diversifies. An investor will not consider that the firm's value is less than P(A)+P(B) for the reason that the firm has combined A and B so that she cannot invest on only A or only B - there are a lot of choices available for her.

So the idea is that P(AB)=P(A)+P(B). The detailed explanation is in Chapter 33:

if P(AB)<P(A)+P(B), investors will not invest on the firms offer only A or B, and vice versa.

金融的基本原理就是分散化投资是有好处的，同样的风险可以有更高的收益。作者的意思应该是如果投资者自己的收入已经分散化了，他就没必要花额外的钱去投资那些多元化的公司来获得好处。后面那段没看明白，通常来说有足够的证券供其分散化投资的话他也不会倾向于投资多元化的公司。

对，我就是后面那句不太明白，前前面那句我能看懂逻辑。谢谢你热心的回答！ 3# uc_sjtu

后一句是说，如果投资者在投资金融证券方面有宽阔而多样的选择，他会尽可能多地投资于证券以完成分散化，而非出之于与之对应的实业，因为他无法在同样成本下投资于多个实业工厂。与前一句是相辅相成的。