希望你们大致记得，这是关于多元函数及其偏导的。
Well, hopefully you kind of vaguely remember that it was about functions of several variables and their partial derivatives.
要寻找等于，减去a的偏导的函数，我们考虑。
A a function of a that looks like the sum of a minus something times the derivative, with respect to that something.
可以想到哪个函数关于x的偏导是，关于y的偏导是x吗？
y Can we think of a function whose derivative with respect to x is y, and derivative with respect to y is x?
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