張宇在好幾年的高數18講的微分方程中講到這個例子。二階可降階微分方程的求解，原則就是：缺什麼就幹掉什麼。缺x就幹掉x，建立p和y的關系；缺y就幹掉y，建立p和x的關系。若不幹掉，就是那種經典的回來複仇的劇情了。
每一個聽過張宇老師課的考研人都明白的
木法沙和辛巴的對話
...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.
宇哥一邊畫圖一邊講這個故事，不小心把辛巴畫成了紙老虎，太有趣了。
他真是一位好老師啊，考研路上有一種精神陪伴着，而且能度過地很快樂，很充實。
将來一定要幫他把萊布尼茲手稿搞到手！
...
