The only place q1 appears here is here, so when I differentiate again I'm going to get -2b and sure enough that's negative, which is what I wanted to know, just to check that when I'm finding this thing, I'm finding a maximum and not a minimum.

只有这一处有q1,因此二阶导数是-2b,它肯定是个负数了,这正是我们想得到的结果,这就验证了我们刚才得出的,是最大值而不是最小值

耶鲁公开课 - 博弈论课程节选