"Senior, that question you asked me in class the other day—why is the coefficient of the impulse response equal to the value of the first-derivative equation?"
"Because the impulse response signal is generated by the second derivative, the original function is continuous, and the jump in the first derivative between $0^+$ and $0^-$ is the coefficient of the impulse response."
*I don't understand a single word of this,* thought Zhang Chenxi, completely baffled.
"You didn't understand?"
Ji Mingyue looked puzzled. To her, this was very basic knowledge that anyone who had taken advanced mathematics should easily grasp. Besides, the advanced mathematics final exam at Longcheng University was quite lenient, with fixed question types. Even someone like Zhang Chenxi, who usually only listened to half of the lectures, managed to score in the nineties.
Zhang Chenxi nodded.
"I'll draw a diagram. It will be easier to understand that way."
Ji Mingyue picked up a pen and began sketching a schematic on a piece of draft paper.
Zhang Chenxi stared at the senior's hand, and the scene from just moments ago flashed back into his mind.
Her hand was as white as mutton-fat jade, crystal-clear, warm, and delicate, as if one could see the veins beneath the skin. When he had held her hand earlier, a strange sensation had washed over him. It was both soft and smooth, like holding a piece of silky satin.
Soon, Ji Mingyue finished the drawing and pointed it out to Zhang Chenxi.
Sitting across from her, Zhang Chenxi found it quite difficult to see properly.
"Senior, where does this $9$ come from?" Zhang Chenxi asked, pointing to a spot on the diagram.
"That's $\delta$, the impulse signal." Ji Mingyue raised her eyes and looked at him expressionlessly.
"Oh, oh, haha! Of course it's an impulse signal! Hehe, I knew it all along, haha, ugh." Zhang Chenxi forced an awkward laugh.
After a few more instances of asking questions that would make even a dog shake its head simply because he couldn't see clearly, Ji Mingyue finally showed a trace of emotional fluctuation.
"You, move over here."
Just as Zhang Chenxi was reeling from another ridiculous question, he heard Ji Mingyue's command-like voice.
At a loss, Zhang Chenxi looked around—there was only one seat on Ji Mingyue's side.
"Bring your chair over, and your books too," Ji Mingyue added.
"Oh, oh, okay."
Suppressing any improper thoughts, Zhang Chenxi carefully carried his chair over and sat down right next to Ji Mingyue.
A fresh and elegant jasmine fragrance emanated from the senior. It was like the first ray of morning sunlight, warm and gentle; yet it was also like a clear mountain spring, babbling and refreshing the soul. Sitting beside her, Zhang Chenxi felt a deep sense of inner peace, and his mind suddenly cleared up.
"Just look over these first, and ask me if there's anything you don't understand," Ji Mingyue instructed before picking up her own book to read.
"Senior... this..."
"Senior... why is this?"
"Senior... where did this formula come from?"
"Senior..."
The book in Ji Mingyue's hands opened, closed, opened again, and closed once more. Finally, she pressed a hand to her forehead, laid her book down, and slid a little closer to Zhang Chenxi.
This junior hadn't listened to a single word in class. If she expected him to review on his own and ask questions just so he wouldn't fail the final exam, she would eat her own words.
...
...
"Senior, can we really do it this way?"
"Mhm."
"Won't this be too fast for us?"
"No. No one will look. We need to hurry."
"But I don't know anything..."
"I'll teach you."
...
...
Finally, after two hours had passed, Zhang Chenxi and Ji Mingyue decided to take a short break.
"Some conclusions can be used directly without derivation. No teacher is going to look closely at the steps; they only care about the result. Professor Li won't make the exam very difficult, and the questions are basically of a few fixed types. Just memorize the solution methods for these problems, and look over a few other knowledge points, and you should be fine."
As Professor Li's prospective graduate student—and having taken his classes herself—Ji Mingyue naturally knew the ins and outs of the course. She picked out a few problems that had a ninety percent chance of appearing on the exam and made Zhang Chenxi memorize their solution methods.
"Take a break, and then try doing them over by yourself."
With that instruction, Ji Mingyue picked up her phone and started browsing.
Having spent two grueling hours looking at problem sets, Zhang Chenxi was just about to open Note to check messages from his fans when a notification popped up from Genie Girl.
[Genie Girl]: "Exams are finally over today! I'm so exhausted, I feel like I didn't do very well." (Crying JPG)
[Puyue Cat]: "Let bygones be bygones. Besides, it's just a feeling; when the results come out, I'm sure it'll be a pleasant surprise."
[Genie Girl]: "That's true too. I am a genie girl after all." (Proud JPG)
Zhang Chenxi muttered inwardly, *How childish. If she were only half as mature and sensible as the senior.*
(Ji Mingyue: "???")
[Genie Girl]: "By the way, you and the senior are in the library right now, aren't you? You promised to let me see what the senior looks like."
[Puyue Cat]: "You mentioned it yesterday, but I don't think I agreed."
[Genie Girl]: "??? We clearly agreed yesterday! Now you're backing out again. All you guys are the same! You just love to cheat me, wuwu. I'm not taking the exam tomorrow!"
*Good grief, your college entrance exam has nothing to do with me! It's not like I'm the one taking it.*
Although Zhang Chenxi was dying to throw his high school homeroom teacher's lectures back at Genie Girl, he figured that once tomorrow's exam was over, he wouldn't have to deal with her nagging anymore. He might as well offer some final words of comfort today.
[Puyue Cat]: "Fine, fine, I'll let you see. But we agreed: I'm only taking a picture from behind, nothing more. No bargaining!"
[Genie Girl]: "Deal! Thanks, Xizhao big bro, you're the best." (Heart hands JPG)
Zhang Chenxi was left speechless. People really do change their faces faster than flipping a book.
[Puyue Cat]: "Do you play Naruto?"
...
...
"Senior, I'm going to the restroom. I'll be right back."
Zhang Chenxi planned to sneak a photo of her back when he returned to shut Genie Girl up once and for all.
Ji Mingyue nodded. Then, she opened her phone's front camera and pointed it in the direction Zhang Chenxi had left.
Soon, Zhang Chenxi returned. He walked quietly to a spot about a dozen meters behind Ji Mingyue, stealthily raised his phone, and prepared to snap a picture of her from behind.
His movements were extremely light, for fear of making the slightest sound and alerting her. His eyes were glued to the phone screen.
*The senior is still playing on her phone. As long as I'm quick, I definitely won't get caught,* Zhang Chenxi thought to himself.
Yet, the exact moment he pressed the capture button, Ji Mingyue—who had witnessed the entire process through her front camera—instantly wiped the smile from her face.
Acting completely nonchalant, she stood up, turned around, and fixed her gaze squarely on Zhang Chenxi, who was currently holding his phone up to photograph her.
Her face was filled with bewilderment and astonishment as she stared at his dilated pupils and slightly parted lips.
Slowly and hesitatingly, she walked toward the frozen Zhang Chenxi.
Zhang Chenxi finally snapped out of it and lowered his phone.
Across that ten-meter distance, as he watched Ji Mingyue step closer and closer, only two words echoed in his mind:
*Ruined.*
Ji Mingyue leaned forward slightly, her eyes revealing a complex mix of emotions—confusion over what had just happened, coupled with a hint of vigilance.
Her voice trembled slightly, her tone laced with incomprehension, fear, and shock:
"Junior, what do you think you're doing?"