"I'm sorry, I can't go for a run today." Chen Ying quickly typed these words on her mobile phone and sent them out.

She didn't have time to reply, so she rushed up the stairs of the dormitory, dragged the suitcase full of homework books from the dormitory, and ran downstairs with all her might. The wheels and stairs at the bottom of the box collided continuously, making a clicking sound. Coming to the door, she pulled out the lever mounted on top of the box, and ran forward with all her might, regardless of the pain in her arm. Her ankles ached from running, and she couldn't breathe. She appeared at the back door of the classroom in a daze, just as the school bell rang in the school building.

She quietly pushed open the door and placed herself and the box in the nearest seat, making as little noise as possible. The teacher has already started to talk about the last point before the mid-term exams, and Chen Yinglai will give a review lesson to the students in the second half of the class. She found the notes in the front pocket of her suitcase and used this last bit of time to browse through them, thinking about the key and difficult points of the exam as she listened to the lecture.

“…… As for the differentiation, by the end of this lesson, all the deductions have been completed - everyone understands it, right? In our midterm exam this time, we only test differential points - although I also talked about a little bit of points last time, but I don't take the test - so everyone use this week to review well, and let's ......take the exam next week in class time."

After saying this, the professor put down the chalk and looked at Chen Ying. She nodded in response to his gaze.

"Then let the teaching assistant give you a review class......" The old professor smiled and picked up his cap and buttoned it into his short white hair, "You can ask her any questions." ”

The bell rang for the end of the first class. The students flocked to the school, rummaging around the desks with their homework books, and surrounded Chen Ying's seat. Chen Ying broke free from the crowd and walked towards the podium with an empty suitcase. She picked up the wireless microphone that the professor had just put down, and blew softly a few times, and the radio came out with a clear exhalation. When most of the students returned to their places, she smiled slightly to cheer herself up, spread out the notes she had written two days ago, and started the review class from the first chapter. She spoke calmly, the more she talked, the more relaxed she became, and the more she talked, the more confident she became. The blackboard that can be pushed up and down is full of writing, and the last corner is also occupied by the board book.

A boy volunteered to go on stage to help her wipe the blackboard.

"Well, just wipe that side." She looked intently at her notes, wondering what else she hadn't said.

"Do you want to wipe both sides off?" A familiar voice sounded around him. She raised her head and looked over, Qin Hong was standing in front of her with a plate eraser, and the middle of her fingers was full of chalk shavings.

"Yes......" she nodded.

He rubbed quickly, white chalk shavings drifting down the blackboard and landing on his shoulders.

"Okay – now let's move on to Rohr's median value theorem. The highlight of our midterm exam is these three median value theorems, which the teacher has already talked about in class last time, do you remember? That's it—Rolle's median theorem, Lagrangian median theorem, and Cauchy's median theorem, these three are mandatory, and at least one big question will come up—" Chen Ying said, grabbing a new piece of chalk and continuing to write fluently on the blackboard, "Everyone must not only understand how these formulas are derived, but also understand which theorem to use when-"

The lesson proceeded in an orderly manner, and the only sound in the classroom was the sound of lectures coming from the loudspeakers and the rustle of pens across paper. From time to time, Chen Ying looked around to see how the students were reacting. Every time she could see Qin Hong sitting in the first row of seats near the door. He kept looking at her with a smile, and from time to time motioned for her to pin the long strand of hair that slipped from her shoulder behind her ear. She gratefully did. She counted the example questions on the blackboard one by one, and every time the blackboard was full, Qin Hong came up to help her wipe it clean. She spoke in a coherent manner until the bell rang for the end of class.

"That's pretty much it—" she concluded, her classmates staring at her intently, "we'll have twenty multiple-choice, ten fill-in-the-blank and eight math questions—we'll have to go back and prepare." ”

The students got up to clean up their things, and the classroom was in a mess. A girl came to the stage with a homework book to ask questions, and after Chen Ying looked at it carefully, she deduced it on the blackboard and told her where she had made a mistake. There were also a few students standing next to them, watching them all the time.

"Sister Teaching Assistant, what if I still can't derive the differential process?" A girl with two pigtails asked timidly.

"Then at least you have to remember the result of the differentiation of each function, otherwise you won't be able to perform the later calculations."

"Senior sister, when will that Lagrange theorem be used? I still don't get it. A tall and thin boy next to him asked, holding a pen.

"The Lagrangian median theorem is a generalization of Rolle's median theorem, first of all, the function must be derivable, and secondly, the definition domain of the function is a closed interval, which can be used if these two basic conditions are met - and Cauchy's median theorem is a re-generalization of Lagrange's theorem."

"Teacher, what is an implicit function?"

"If you go back and take a closer look at the concept in chapter 3, verse 3 of the book, it is very clear what an implicit function is. It's also best to do the following questions to deepen the impression. ”

"Senior sister, I didn't find my homework book." A confused-looking boy said.

"But I've already brought all the homework books, can you go over there and have a look?"