“Solve this inequality.” Seohyun pushes the paper towards Yoona, who looks at it, and then at her with a confused expression.

9x-7i > 3(3x-7u)

“This is so simple. I don’t—”

“Please, unnie.”

Yoona caves; it’s hard not to in the face of that earnest expression. No one can make an honorific sound…appealing the way Seohyun does.

“You even have me doing math in my spare time now. What do you do to me?”

Seohyun just smiles that quiet, private smile of hers, watching Yoona like she could look at her doing inequalities for the rest of the day.

It doesn’t take Yoona very long to solve it, even though just a few months ago she would have forgotten to reverse the sign of the inequality when dividing by a negative. (Seohyun is a brilliant tutor, and Yoona’s not just saying that because she’s biased.)

“3u is greater than i – did I do that right?” Yoona looks at Seohyun expectantly.

Seohyun clears . “Reverse the sign.”

“What? But I thought—”

“I mean, express the answer as a less than rather than a greater than.”

“Okay?” Yoona says slowly, and she simply switches u and i and the direction of the sign. “i is less than 3u. It’s the same thing.”

“Unnie.” Seohyun sounds pained, like Yoona’s missing something obvious and important.

“I don’t get it,” Yoona says slowly. “Did I do something wrong?”

Seohyun takes her pencil from her and erases the 3 before writing it closer to the less than sign rather than the u.

“This is the right answer.”

i <3 u

Yoona stares at that for a few seconds, or maybe a few minutes, and then she looks at Seohyun, who’s watching her attentively, almost anxiously.

“Come here,” Yoona says softly, pulling Seohyun to her. Seohyun is slightly tense at first, but she relaxes almost immediately in Yoona’s arms, turning soft and pliant against her. “You’re too cute.”

“I don’t – I’m good at math, but I’m not good at other things,” Seohyun stumbles through her words. “I’m not expressive like Tiffany unnie, or romantic like Yuri unnie—”

“You’ve been doing too many inequalities,” Yoona cuts that train of conversation off gently but firmly. “Stop comparing yourself to other people. I like you for you, just the way you are.”

Seohyun looks at her the way Yoona looked at her first aced math test, happy but a little disbelieving, like this is too good to be true and she’s afraid someone will snatch it away from her.

Yoona smiles at her and squeezes her hand. She doesn’t understand how someone as smart as Seohyun can miss how Yoona feels about her, but then again, Seohyun seems to understand the relationship between functions better than the relationships between people a lot of the times.

Yoona will just have to help her with that, then.

“This is the sweetest inequality I’ve ever done,” she says with a smile. “It’s a lot better than the stuff Sica unnie told me.”

“What stuff?” Seohyun asks curiously.

“You know, like – I wish I was your derivative so I could lie tangent to your curves,” Yoona says casually, “or I’ll take you to your limit if you show me your end behaviour.”

“Unnie!” Seohyun’s face is bright red. Yoona doesn’t think she’s ever seen anything so cute. “That’s, that’s just—”

“Creative?” Yoona supplies innocently.

“I didn’t know Jessica unnie knew things like this.”

“She learned from the best.” Yoona shrugs. “I guess Taeyeon unnie got her nickname for a reason.”

Seohyun’s still blushing, but more of a pink than red. Yoona thinks that she’s enjoying this a bit too much, and she wonders if Jessica’s ersion has rubbed off on her.

“I’m all out of pick-up lines, so I guess I should just ask you directly.” Yoona smiles. “If I can kiss you, I mean.”

“You don’t need to ask,” Seohyun says, leaning closer to Yoona hesitantly, like she’s the one waiting for permission. Yoona closes the last bit of distance between them and gently presses her lips to Seohyun’s, expecting her to kiss back tentatively.

She doesn’t. There is nothing tentative or shy about the way Seohyun kisses her. Yoona doesn’t know where she learned to use her tongue like that, but she sure isn’t complaining.

“Unnie,” Seohyun says breathlessly.

“If you’re going to talk about the upcoming test, I swear, I’m going to—”

“No, no, I wasn’t. I was just wondering if you wanted to go to your room, since there’s more space there.”

Yoona stares at Seohyun, wondering if her ears are working right. Seohyun can’t possibly be insinuating what Yoona thinks she is, can she?

“The papers are so cluttered here,” Seohyun says. “I brought a second textbook because the theorems are better there, and I don’t know if we can fit everything.”

“I thought you said you weren’t going to talk about math.”

“But we can kiss and study at the same time.”

Yoona doesn’t know whether to laugh or—actually, she just doesn’t know. Seohyun is so…Seohyun. Yoona may be a lot better at languages than math, but she doesn’t think there are enough words in the world to really capture Seohyun.

“The only thing I want to study right now is your mouth,” Yoona says. “And I came up with that line all by myself. I think I should get a prize for that.”

“You’re very creative, unnie,” Seohyun says solemnly, and then she’s kissing Yoona again, and Yoona doesn’t have enough mental capacity to think of a comeback.

It’s all right; she’ll save her brainpower for studying. If this is practice, then she can’t wait for the final exam.

Later on, while Seohyun is combing her fingers through her hair, Yoona remembers a line that she actually kind of liked.

“If I were a function you would be my asymptote – I always tend towards you.”

Seohyun smiles at that, and then a thoughtful expression crosses her face, pensive and a little sad. “But functions never touch their asymptotes. They get closer and closer, but they never reach their asymptote.”

Yoona reaches for Seohyun’s hand and winds their fingers together. “Math is full of tragic love stories, it seems. Tangents – you only meet once and then part forever. Parallel lines – you follow the same path but they never intersect. Asymptotes – you come closer and closer but you never touch.” She looks at Seohyun. “What kind of lines do you think we would be?”

Seohyun looks like she’s thinking very hard about the answer to that question. “I don’t know,” she finally says. “I just know that I want to be with you.”

Yoona smiles and tucks her head against Seohyun’s shoulder. She’s more than happy with that answer.

Yoona may have been the one to start the pick-up lines, but Seohyun takes to them with a seriousness that is both surprising and amusing. Yoona doesn’t know if she likes them because it’s easier to be sentimental under the guise of calculus and geometry, or if Seohyun’s just that much of a math nerd.

Yoona would see sticky-notes in the margins of her notebook, under graphs in her textbook, even between the back of her calculator and the cover, with things like I’m irrational around you, or Your beauty cannot be spanned by a finite basis of vectors, and they always make her smile, even though Jessica never stops with her teasing if she sees one.

Seohyun never uses a line with the word “love” in it though (and Yoona knows there are a lot), and she’s never said it other than that one time when Yoona fell asleep after four hours of studying optimization and woke up bleary and drowsy to hear Seohyun whispering the words, so quietly that Yoona wonders if she imagined them or if she’s still dreaming.

Yoona doesn’t need Seohyun to tell her those words though; she doesn’t need to hear them to know them. She still can’t help wanting to hear them, but she’s willing to wait. She’s not a very patient person, but Seohyun is worth waiting for.

“Look at you,” Yuri says warmly. “You’re glowing.”

“It’s these new lights they installed here,” Yoona says. “They do wonders for my complexion.”

“They don’t do anything for Yuri’s,” Jessica says. “They just make her look darker.”

“Hey! You said you liked my tan.” Yuri sounds wounded.

“I said I liked your tan. I didn’t say you could go to the beach without sunscreen and end up looking like a cooked lobster.”

“I forgot it, okay? I didn’t do it on purpose.”

Yoona looks fondly between the two of them. “You bicker like a married couple.”

“Ugh, no thanks,” Jessica says. “Marriage is too much work.”

“Everything is too much work to you,” Yuri snorts, “except sleeping.”

“Or something else done in bed,” Jessica says casually, and Yuri looks like her face catches fire.

Yoona clears . “I don’t need to know about your life, thank you.”

“How’s your life going, Yoong?” Jessica asks. “Do you think you’ll have one any time soon?”

“Jessica,” Yuri hisses. “You can’t just ask her that.”

“Why not? I’m just being a good friend.”

“There are boundaries that friends should respect.”

“Hey, I’m totally being respectful. I’m asking Yoong upfront rather than talking about her behind her back.” Jessica turns to her. “So, Yoona – have you gotten to second base yet?”

Yoona hears herself say, “We’re not having this conversation,” like listening to someone else talk.

“Don’t be shy, Yoong.” Jessica clucks her tongue. “Have you not gotten the talk about the birds and the bees yet?”

“I’m leaving now.”

“Be safe!” Jessica yells at Yoona right before she’s out the door.

“Hey, Yoona!” Tiffany’s smile almost blinds Yoona; she thinks she might need to start wearing sunglasses around Tiffany. Maybe some earbuds too. “How’s it going?”

“Good.”

“I heard TaeTae gave Jessi a list of pick-up lines, and she gave them to you.” Tiffany looks disapproving. “Those were terrible. I hope you didn’t use the one about the bed and the clothes.”

It takes Yoona a few seconds to realize which one Tiffany is talking about. is like math: add the bed, subtract the clothes, divide the legs and pray you don’t multiply.

“Oh, no no, of course not.” She shakes her head vigorously. “And anyway, we can’t multiply,” she adds as an afterthought. “We’re both girls.”

Tiffany giggles, her eyes disappearing into crescents. “TaeTae and her pick-up lines,” she sighs. “I don’t know why she even tried to use them on me. I barely understood half of them.”

Yoona grins. “Did she end up having to explain them to you?”

“Yeah. At least she was patient about it. Now she always makes fun of me when I ask her what something means.”

“Really? But she’s crazy about you.”

“Jessi says that making fun of me is her way of showing affection.”

Yoona snorts. Of course Jessica would say something like that. If teasing equals showing affection, then Yuri would be drowning in Jessica’s love.

“Sica unnie seems to think she’s some kind of love guru.”

Tiffany laughs so hard she doubles over. “I’m sure she’s just looking out for you,” she says, once she’s recovered from her bout of laughter.

“Of course she is.” If looking out equated to teasing. “That’s what best friends are for.”

“Yes, that’s true.” Tiffany’s expression turns solemn. “Please be good to mine.”

Yoona nods, although of course she would regardless of whether Tiffany asked her to or not. Seohyun means a lot to her, more than Yoona ever thought she would that first “What are you struggling with especially?” day.

The memory makes her smile, and she realizes that Tiffany is looking at her intently, probably thinking that she’s crazy for grinning out of nowhere.

Tiffany beams. “You’re so cute, Yoong,” she declares, pinching Yoona’s cheek. “I can call you Yoong, right?”

Tiffany is all wide eyes and bright teeth. Yoona nods dumbly, wondering why all her unnies seem younger than her.

“Your best friends scare me,” Yoona tells Seohyun while watching a Pirates of the Caribbean movie with her. Seohyun likes Johnny Depp so much that Yoona wonders if she should be jealous.

Seohyun gives her a puzzled look. “But Tiffany unnie is so sweet. And Taeyeon unnie’s…not very intimidating.”

“You said you were scared of Sica unnie.”

“But her expression can be…very cold.” Seohyun looks at Yoona like she wants her to think of a better word.

“You mean, she has a very impressive face.”

“Maybe not that exact word,” Seohyun hedges, “but she can be—intimidating.”

“Okay, fine, maybe at first,” Yoona concedes, “but she’s really a softie once you get to know her.”

“I’ll try,” Seohyun promises, and Yoona knows she’ll put as much effort into it as studying for a final. Seohyun’s like that, admirably and stubbornly dedicated, and Yoona likes that about her.

“Geez, I feel like we’re both trying to impress the in-laws or something.” Yoona stretches out her legs, resting her feet on the coffee table, but with a glance at Seohyun, she puts them back on the floor.

Seohyun looks sheepish. “You can do that, unnie. It’s okay.”

“You don’t like it.”

“Taeyeon unnie says I can be too…stiff.” Seohyun bites her lip and looks at Yoona. “Unnie, do you think I’m stiff?”

Yoona puts her hand between Seohyun’s shoulder blades, spreading out her fingers and pressing her palm down. Seohyun makes a faint motion that might be a twitch or a shiver, and Yoona’s mouth pulls up at the corners.

“You’re kind of tense here,” she murmurs. “You have this knot right in the middle. It makes me wonder what I can do to get rid of it.”

Seohyun looks at her, slightly wide-eyed. “A massage?”

Yoona pauses. “Do you want a massage? I actually give pretty mean back rubs.” She cracks her knuckles to demonstrate the strength in her fingers (hey, she was once given the nickname ‘Him Yoona’ for a reason.)

“Maybe another day,” Seohyun says, eyes returning to the TV. “I want to watch Johnny Depp right now.”

Yoona almost pouts, but she manages to hold it in.

Seohyun takes her hand and winds their fingers together. “You shouldn’t crack your knuckles so much, unnie. That’s not good for your joints.”

“You sound like I’m an arthritic old lady.”

Seohyun’s eyes widen, and Yoona already has a soothing reply on the tip of her tongue, when Seohyun unexpectedly says, “I just don’t want you to end up as one in your senior years.”

“Seohyun!” Yoona starts laughing, full-bodied and warm, starting in her stomach and reverberating in her chest. She can’t even tell if Seohyun is being sarcastic or not, and that just makes it funnier. “God, I love you so much.”

The words may have slipped out, and she may still be half-laughing, but she means them with all of her heart. She can tell Seohyun knows that, because Seohyun ducks her head and smiles a sweet, still slightly shy smile that makes Yoona want to cuddle her and kiss her at the same time.

“Unnie,” Seohyun starts, looking almost painfully earnest.

“It’s okay,” Yoona says gently. “I can wait. My patience is like a vertical asymptote.” Seohyun gives her a blank look, and she clarifies, “It’s limitless.”

Seohyun laughs. “Your math puns are…” She looks like she’s struggling to think of a word that isn’t too much of an insult.

“Something else?”

“Something else,” Seohyun agrees. “You’re something else.”

Her eyes are soft and liquid and Yoona would rather look into them than Johnny Depp’s or Jo Insung’s any day.

“Thank you. I try to be.”

Seohyun smiles. “You don’t need to try.”

And afterwards, Yoona puts her head on Seohyun’s shoulder, content and comfortable, and Seohyun goes back to watching her precious Johnny Depp. She’s warm and solid against Yoona, and she smells like sea island cotton lotion and some vaguely floral shampoo, and—it’s just nice, to be with her like this. A different kind of nice from winning a gruelling soccer match, or acing a test she worried about, or even eating her favourite meal.

A Seohyun kind of nice, Yoona decides. She’ll just have to use Seohyun’s name for the superlatives she doesn’t know how to express.

Yoona ends up sleeping right through the rest of the movie and most of the evening. At about eight Seohyun shakes her awake – gently, apologetically – and asks her if she wants some dinner.

“Oh, dinner.” Yoona perks up. “Sure, I could go for some food.”

“Tiffany unnie says that my food is pretty good, just kind of bland,” Seohyun tells Yoona, her head poked in the fridge, an apron with pink flowers all over it tied around her waist.

“Did she get you that apron?”

“Yes?” Seohyun sounds confused at the question. “How can you tell?”

“All the pink,” Yoona says simply.

“Ah.”

“She’s almost like your mom, isn’t she?”

“She’s a good unnie,” Seohyun says. “She takes care of me a lot, especially since…” The line of her shoulders looks almost painfully sharp when she shrugs. “Well, my mom is busy. She doesn’t always have time for me. Tiffany unnie knows what it’s like to want a mom around.”

Yoona doesn’t have anything to say to that. She’s noticed that out of all the times she’s gone to Seohyun’s house, she’s only met her dad once, when he was heading out, and she’s never even met Seohyun’s mom. Yoona’s parents wouldn’t be winning any parenting awards any time soon, but at least they take good care of her.

She wonders if the reason Seohyun is so level-headed and mature is because she was forced to grow up quickly.

“Am I a good unnie?” Yoona asks lightheartedly.

“Of course, but not in the same way as Fany unnie.”

“Why not?”

“Because.” Seohyun turns around to smile at her, a pack of mushrooms in hand. “Fany unnie is…an older sister figure, and you’re…you. You know you’re my favourite unnie.”

“Yeah?” Yoona can’t contain her ear-splitting grin at that. “I still am?”

Seohyun’s smile softens, and her eyes crinkle at the corners. “Always,” she says, quiet but firm, and Yoona smiles, not wide but full of the happiness her heart can hold, which is not an amount she would be able to express in numbers. She’s gotten better at math – she has the best tutor ever, after all – but she’s not that good.

A/N: Two other pick-up lines I wanted to incorporate into the fic but couldn't (technically three including the one in the summary):

The derivative of my love for you is 0, because my love for you is constant.

Our love is like dividing by zero... you cannot define it.