张宇在好几年的高数18讲的微分方程中讲到这个例子。二阶可降阶微分方程的求解，原则就是：缺什么就干掉什么。缺x就干掉x，建立p和y的关系；缺y就干掉y，建立p和x的关系。若不干掉，就是那种经典的回来复仇的剧情了。
每一个听过张宇老师课的考研人都明白的
木法沙和辛巴的对话
...Mufasa:Simba,you have forgot me.Simba:No,how could I?
Mufasa:You have forgot who you are,and you've forgot me.Look inside yourself Simba, you're more than what you have become. You must take your place in the circle of life.
Simba:How can I go back?I'm not I used to be.
Mufasa:Remember who you are,you are my son,and the one true king.Remember who you are.
宇哥一边画图一边讲这个故事，不小心把辛巴画成了纸老虎，太有趣了。
他真是一位好老师啊，考研路上有一种精神陪伴着，而且能度过地很快乐，很充实。
将来一定要帮他把莱布尼兹手稿搞到手！
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