Back in 1978, I was admitted to Northwestern Polytechnical University.

Chapter 22 I am a substitute teacher
Is this Qin Liang?

Yeah, I only glanced at it a few times during the class meeting on the first day of school, and I didn't remember it. Now everyone is recognizing it!

Skipping class for a week is definitely a record for Class 5381, and I wonder if anyone else will break this record!
The people around them had complex emotions: envy, worry, sympathy, and disdain...

Wait a minute, Qin Liang isn't heading towards his seat, he's heading towards... the podium!
"Class begins!" Qin Liang put down his lecture notes and looked down at the classroom.

There was no response! Shouldn't they be saying "stand up, please" at this point? Didn't Teacher Zhao say these things in class?

"Qin Liang, come down!" Zhang Yong said.

“That’s right, Qin Liang, you should get down here right now. If Teacher Zhao comes in and sees you taking his seat, he’ll be unhappy!” Zhao Zhijian added.

Qin Liang was helpless.

Do you think I wanted to stand on this platform? I was forced to! Teacher Zhao suddenly developed a high fever last night. This morning, as soon as the fever subsided, she rode her bicycle to class, but... she fell halfway there and got a head injury.

At that time, Qin Liang happened to be passing by, and then... in order to put Teacher Zhao at ease, he had no choice but to agree to take the lecture notes and teach the class himself. Otherwise, Teacher Zhao planned to go to the school hospital for treatment and then continue teaching!
If it's another course, you can find another teacher to teach it. This is a purely English textbook, and there are very few teachers in the entire aircraft department who can teach it!
Come on, I've been doing calculations for a week straight and my head is spinning. I'll just treat this class as a way to relax!

"Alright, quiet down. Teacher Zhao isn't feeling well, so I'll be teaching today's class." Qin Liang switched to Chinese: "Now, open your textbooks to Chapter Two. We have quite a bit of material to cover today, so my pace might be a bit fast. If you don't understand something, feel free to raise your hand. Now, let's continue with the English lesson."

"Derivative, the mean is to lead or draw off..."

His pronunciation was clear and standard, even more fluent than the senior students in the dormitory. It was just like hearing the sound of a cassette tape on my own Sunflower brand tape recorder!
The others also quieted down. They knew they had to listen carefully to understand. Even though it wasn't Teacher Zhao giving the lesson, they had to listen attentively no matter who was speaking!

Half an hour passed, an hour passed, an hour and a half passed...

Finally, the school bell rang outside.

"Alright, everyone, we'll have twenty minutes of activity time. After that, we'll continue with the second half of this chapter."

A group of people crowded around the podium.

"Qin Liang, did you study English in high school?"

"Qin Liang, when did you start learning advanced mathematics?"

"Qin Liang, you're amazing! You're number one in our class, you're really awesome!"

Zhang Yong and Zhao Zhijian couldn't squeeze in from the outside of the crowd. They were still holding their textbooks, and they hadn't understood part of what they had just heard!

If it were Teacher Zhao's class, they would have rushed over long ago, but... after saying bad things about Qin Liang in the morning, they were too embarrassed to ask him questions now. So, in that moment of hesitation, they missed their chance!

"Excuse me, everyone! Can't you hear me? Teacher Qin's voice is hoarse!"

Everyone made way for Zhao Xiaoxia, who came up carrying a white enamel mug. The mug was printed with the four characters "Gong Cheng Yong Yi" and a four-digit code.

"Teacher Qin, have some water. There are a few dried sea buckthorn berries soaked in it, which are very good for your throat."

Qin Liang was indeed thirsty, and without thinking much of it, he took the cup and gulped it down…

A girl nearby couldn't help but exclaim, "Xiaoxia, that's your own cup, you..."

"There's no need for such formalities. Teacher Qin just needs to moisten her throat so she can continue teaching us." Zhao Xiaoxia readily accepted the cup. "Is that enough? Would you like another cup?" "No, that's enough."

"Okay, then tell us, what are the practical uses of the derivative?"

The most important thing is to apply what you've learned; how do you connect the content in textbooks with real-world situations?
"Yes, advanced mathematics is an effective tool for our research. Take kinematics, which we learned in high school, for example. The velocity of an object is actually the first derivative of its position function, and the acceleration is the first derivative of the velocity. You can use the knowledge you just learned to take the first derivative of the displacement formula from high school and see if it becomes the velocity formula..."

Everyone quickly got to work, and sure enough!

"Furthermore, in geometry, finding the slope and finding the optimal solution in various graphical analyses all require the use of derivatives..."

Qin Liang's words enlightened everyone.

"Qin Liang, there are a few things I don't understand from what we just covered..."

As soon as the voice rang out, everyone looked behind them.

"Zhang Yong, Qin Liang is our teacher now, you have to call him Teacher Qin!"

"That's right, his lesson was really good. No wonder he doesn't come to class anymore. It turns out he's already good enough to be our teacher!"

Zhang Yong blushed, but glancing at the unresolved questions in his textbook, he still managed to speak: "Qin..."

"Teacher Qin, please explain it to me first!" Zhao Zhijian said naturally, handing over the textbook as he did so.

Confucius said, "When three people walk together, one of them will be my teacher." What's wrong with calling someone "teacher" if they are more capable than you? Why are you even embarrassed to call them that? Zhao Zhijian looked down on his roommate.

The morning classes ended, and unsurprisingly, Qin Liang was surrounded by a group of classmates again. By the time he finished answering their questions, forty minutes had passed!
"Teacher Qin, the cafeteria is sold out at this time. How about I treat you to something at one of the small restaurants near the school gate? Consider it a thank you for teaching us," Zhao Xiaoxia said as she approached after everyone had left.

She was indeed very curious about the secrets surrounding Qin Liang!
"No need, someone will bring my food." Qin Liang waved his hand. "Goodbye!"

Someone delivered it?
Zhao Xiaoxia asked curiously, "Who delivered it? Was it your roommate?"

"No."

Who is that?
Before Zhao Xiaoxia could ask the question, she saw Qin Liang riding away on Teacher Zhao's big 28-inch bicycle, creaking and groaning. Judging from the direction, it was the Fifth Department office building behind Teaching Building No. 1.

Watching Qin Liang's retreating figure, Zhao Xiaoxia decided to follow him to see what he was up to at school.

Office building, double-delta wing project office.

"Wow, not bad! Steamed pork ribs, celery and sliced ​​pork. How come our lunch is so plentiful today?" Qin Liang asked, looking at the leftovers for him.

"Old Zhao instructed that, 'Thanks to you, otherwise the students' morning would have been wasted.'" Professor Sha's voice rang out. "He specifically asked for two extra dishes for you, to be on his tab."

"Teacher Zhao is too kind!" Qin Liang said, picking up his chopsticks. It was such a rare treat to be able to eat meat every day in 78.

The office door slowly opened a crack.

(End of this chapter)