We're asking the question, taking S2 as given, what S1 maximizes this expression and as the gentleman at the back said, I'm going to differentiate and then I'm going to set the thing equal to 0.

我们来讨论在S2已知的情况下,S1的最大值是多少,就像坐在后排的先生说的那样,我们要去求导然后令导数等于0

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