The noise between classes was like a boiling pot—boys shouting about basketball games, girls laughing and sharing snacks, all mixed in with the buzzing of the ceiling fan, filling the classroom to the brim. Lin Chen had just finished marking the "common mistakes in function monotonicity" in his error notebook when, out of the corner of his eye, he spotted Su Wanqing pacing nervously beside her seat, clutching her math practice book. Her white canvas shoes scuffed against the floor, and the book's cover was almost creased from her nervous grip. It was clear she was struggling with a tough decision.
Suddenly, Su Wanqing took a deep breath, like a little rabbit gathering all her courage, and briskly walked over to Lin Chen's desk. Her ears were as red as cherries, and her voice was so soft it barely registered above a whisper, but Lin Chen could hear every word crystal clear: "Lin Chen, the monthly exam is next week, and I still don't get the monotonicity of functions... can you, um, help me explain it?"
Lin Chen's pen slipped from his hand and hit the desk with a soft clink. His heart surged with joy—this was the first time Su Wanqing had come to him for help! He immediately nodded, his voice a bit higher than usual in his excitement: "Of course! I'll explain it now, and I promise you'll understand!" He quickly pulled out a piece of scrap paper, grabbed a ruler and red pen, and started sketching a mind map, completely ignoring the murderous glares coming from the back row.
Zhao Kai, sitting behind them, saw the scene and gripped his pen so hard that it creaked. His face darkened, turning as black as the bottom of a pot. He had originally planned to pick on Lin Chen, but now Su Wanqing had approached him herself. His anger flared up like gasoline on a fire, his teeth clenched in a silent snarl: Lin Chen, you dare get this close to Su Wanqing? Just wait until the monthly exam. I'll make sure to make you regret this!
"See," Lin Chen continued, pointing at the graph of a function on his scrap paper, drawing red lines on the key steps, "the core of monotonicity is judging the sign of the derivative!" He was so into explaining that he didn't notice Su Wanqing leaning in closer, her nose almost touching the paper, her eyes glued to the steps. Her long eyelashes fluttered slightly with each blink. Sunlight streamed through the window, casting a warm glow on the tips of her hair, which shimmered with a golden light. Lin Chen couldn't help but soften his voice as he looked at her focused expression.
"I think I'm starting to get it!" Su Wanqing suddenly lifted her head, her eyes shining with excitement. "What about this one? f(x)=lnx - x, how do I find the monotonic intervals? I couldn't get it right when I tried last night." She pointed at a problem in her practice book, her voice full of hope.
Lin Chen quickly took the practice book from her, glanced at the problem, and began to explain. His pen smoothly danced across the paper: "First, the domain is x > 0 because of the ln function's domain restriction. Then, we find the derivative, f'(x)=1/x - 1, which simplifies to (1 - x)/x. Now, to check the sign: since x > 0, the denominator is always positive, so we only need to check the numerator. When 1 - x > 0, x < 1, meaning the function is increasing on (0, 1); when 1 - x < 0, x > 1, meaning the function is decreasing on (1, ∞). Simple, right?"
Su Wanqing's eyes lit up, and her face brightened with realization. "Oh, I see now! I forgot to simplify it yesterday—no wonder I couldn't get it right!" She pulled a beautifully wrapped chocolate from her bag, quickly pressed it into Lin Chen's hand, "Here, for you! Thanks for helping me out, it's so delicious!"
Lin Chen took the chocolate, his fingers brushing against hers. The warmth shot through him like electricity, and his ears turned red. "You don't need to thank me. We're desk partners, helping each other out is normal!"
The bell for the next class rang, cutting off their moment. Su Wanqing quickly returned to her seat, but not before turning back and silently mouthing "We'll talk more after class" with a smile that was more radiant than the morning sun. Lin Chen watched her walk away, a warmth spreading through his chest. Even the dull history class that followed seemed more interesting now.
After the bell rang for the break, Lin Chen eagerly walked up to Su Wanqing's desk, holding the carefully prepared "common mistakes in function monotonicity" draft. "I was worried you might forget some of the details, so I made a list of the key points, like double-checking the domain after differentiation and making sure you don't forget the denominator's square in fractional derivatives…"
Su Wanqing looked at the draft, but her expression grew slightly worried. "But we only have ten minutes between classes, and I still don't get a few other problems... What if I don't do well on the monthly exam? My mom will definitely yell at me," she said, the worry evident in her voice.
Lin Chen's heart skipped a beat. Seizing the moment, he quickly suggested, "How about this: during lunch break, we can go to the library? It's quiet there, no distractions, and we can go over a few more problems. We can really nail down the key points."
Su Wanqing's eyes lit up, and she eagerly nodded. "Yes! Let's meet at the library entrance after lunch, and don't you dare leave without me!" Her ears turned bright red again as she quickly packed up her practice book, but the smile on her face couldn't be hidden.
After they made the plans, Lin Chen headed back to his seat, excitement bubbling up inside him. He knew how rare these library moments were—an opportunity to help Su Wanqing with her math while also getting closer to her. Little did he know, in the back row, Zhao Kai was watching them with a malicious grin. He had already made his plans: during the exam, he was going to sneakily swap Lin Chen's answer sheet, making sure he bombed the test. That way, he could see if Su Wanqing would still stick close to him after the disaster.
During lunch break, Lin Chen and Su Wanqing met up as planned and made their way to the library. Su Wanqing, carrying her pink backpack and clutching her practice book, looked like she was glowing with happiness. "Let's sit by the window. I saw last time that the light there is perfect."
"Sounds great, you pick the spot," Lin Chen said, smiling as he followed her into the library. The sunlight filtered through the windows, bathing the tables in a warm glow. Lin Chen pulled out his draft paper and began explaining more function problems to Su Wanqing, from basic types to the trickiest pitfalls, making sure to go step by step. Su Wanqing listened attentively, occasionally asking questions, and they worked seamlessly together. They lost track of time as they immersed themselves in their math session.
As the librarian reminded them that the next class was starting, they hurriedly packed up. Su Wanqing looked at the dense notes on the paper with gratitude in her eyes. "Lin Chen, with your help, I'm sure I'll do great on the monthly exam!"
"Of course!" Lin Chen replied with a confident smile, walking her to the entrance of the teaching building. He watched as she disappeared down the hallway, his heart filled with anticipation for what was to come. But little did he know, a scheme against him was quietly taking shape—Zhao Kai had already gotten in touch with an exam proctor he knew, planning to sabotage Lin Chen during the monthly exam and make him fail spectacularly.