Source
https://kakuyomu.jp/works/16818093093605365078/episodes/16818622173225097846
“—So, this sigma is the sum of all the numbers in the number line.”
“That’s what it means !? That’s so simple !”
Shaking off all sorts of temptations, we started studying maths.
Ayame san taught me the other day, so teaching Riria would also help me review the material. This might kill two birds with one stone.
“Oh, so that’s what sigma means.”
“You know…the teacher kept telling you that in math class…don’t you remember?”
“Well…..I wasn’t really listening….”
“…..I was stupid to ask.”
“But it’s easier if you know what it means. The point is to add up all these numbers, right?”
“Well, that’s it.”
“But why do you use confusing letters for this? When I saw it, it looked like a bag of bread.”
“Well, it does look like that.”
“By the way, it’s called a ‘bread bag clip’.”
“How do you know the name of the bread bag clip when you don’t know what it means……”
“Well, it’s just like the English word Bag Clip would be more fitting.”
“I love how your English pronunciation is for nothing !”
While being tossed around by Riria’s unexpected special skills, I proceeded with my learning.
It is a great progress to know the meaning of sigma in the problem of the number line. Knowing this makes a real difference.
“All right, let’s see, add one, add two, add three……”
“Um, Sotogahama san?”
“What?”
“I mean, are you, by any chance, trying to add it all up?”
“Yeah, so……? Because that’s what you do when you add up all the numbers in a sequence, isn’t it?”
“Who would honestly add them all up !”
Why do we use the brainwave method of adding one by one when we are trying to find the sum in mathematics……
No, it’s true that if you take the time to calculate without mistakes, you can definitely arrive at the correct answer.
Perhaps this woman is asking for probability calculations by writing out all the steps?
If she did that during the test, she would be overwhelmingly short on time. Even if there was a button for 100 million years, it would not be enough.
“You know, there is an efficient way to calculate this.”
“You need to tell me that asap.”
“Some of us notice this much, but…”
“I’m an idiot anyway !”
“Alright, alright, I’ll explain it in a way that even a moron can understand, so please listen carefully.”
This time, the sequence is very simple: the first term is 1, the common difference is also 1, and the number of terms is 99.
In short, it is a sequence of integers from 1 to 99.
The problem is to add all the integers from 1 to 99.
It is not wrong to add all of them as Riria did, but there is a smarter way to do it.
“Sotogahama san, what if you add one and ninety-nine?”
“Are you making fun of me? Even I can tell that it is one hundred.”
“That’s right. Then what about two and ninety-eight?”
“That’s a hundred, too.”
“Then three and ninety-seven.”
“A hundred.”
At this point, Riria said, “Ah.” It seems she noticed something.
“—-Here, if you add the front and back of the number line, you get a whole lot of hundreds !”
“Oh, good point.”
“If you keep repeating this, you’ll get a hundred…..forty-nine ! And we’ll have one fifty left over !”
“Right. So add them all up?”
“Four thousand nine hundred and fifty ! That’s amazing, Hirakawa kun !”
“The formula for that method is the one in the textbooks.”
“Oh, oh, oh ! ! ! I feel like I can do math now !”
Riria is leaking brain juice and looks like she’s feeling good.
It feels good to be able to notice and solve problems on your own, doesn’t it? It’s different from the feeling of doing something naughty. I understand.
After that, Riria continued to study math, solving problems as I explained the meanings of letters and symbols, like the moment the long stick in Tetris arrived.
Perhaps she is not inherently dumb, but rather a person with good sense.
But when she gets stuck on something, she stops going any further. That is the type of person she is.
So, when I explain to her what she doesn’t understand, she moves forward in an interesting way.
I see, I see. This makes teaching fun, too……!
“–Amazing, I solved all the practice problems in the textbook…!”
“You’re doing a great job, Sotogahama san. This will not only help you avoid repeating the year, it will even help you get to the top of the class.”
“Ehehe…I guess I can do it if I try. Mmhmm.”
“Okay, let’s keep this up. Not just math, but chemistry next time.”
“Well, if we cram so much in at once, I’ll lose all the stuff I learned in the beginning, like the number line.”
“Do you have short-term memory loss….”
Well, it’s true that we’ve been studying for about two hours straight, so it’s okay to take a break here and there.
Even idiots say it’s time to take a break. I don’t know, though.
“Then, let’s take a break. Let me get you something to drink.”
“Thanks !”
I went to the kitchen and opened the refrigerator door.
As I recall, Riria is a fan of soda. I thought it would be a good idea to bring a can of Mountain Dew, which I have in stock.
I grabbed two cans from the back of the cabinet.
—They’re cold–so cold…..! Thank goodness…..!
With that, I return upstairs to my room and slam open the door.
For some reason, Riria looked back at me with a surprised look on her face.
She was holding something in her hand, but she was hiding it so I couldn’t see.
It looks……suspicious…..?
“W-welcome back Hirakawa kun…..”
“Sotogahama san……? What were you doing?”
“N-no……it’s nothing, okay?”
“Suspicious !”
I was curious to see what Riria had in her hand, so I peeked at her hand.
However, Riria also doesn’t want to be found out, so she hides it.
Not to be outdone, I pursued further.
Then, I see a cookie tin in Riria’s hand.
Ah, that cookie tin contains…that thing.
It’s bad if she sees it.
“……Sotogahama san? You saw the contents of that cookie tin……didn’t you?”
“No, I didn’t. I didn’t see it at all.”
“I see….so you haven’t.”
“Y-yeah, I wouldn’t look at it without permission……right?”
“Oh…by the way, how many millimeters was it?”
“It’s rare, something as thick as 0.1 mm…ah.”
That is such a classic way to be exposed.
Perhaps she was hungry and suddenly put her hand in the cookie tin. It was my mistake to hide it there.
Oh…..what an excuse……
As I was thinking about that, I was taken by surprise and pushed onto the bed by Riria.
“You were really expecting something after all, weren’t you? Otherwise you wouldn’t have hidden something like that, right?”
“W-wait a sec, I didn’t buy that thing…….”
“I don’t care about that. It’s rare to have something so thick, and I’m curious about how it feels to use. Let’s give it a try.”
“Y-you said you decided to study today…….”
“This is also part of studying ! Health and physical education study !”
While wondering about that stupid excuse, I was unable to stop Riria who had already been turned on, and she began to take off her clothes.
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