整数の性質は大学入試数学Aの超重要分野だよ！約数・倍数・素数の概念は、不定方程式などの他の分野を解く上での基礎になるから、ここでしっかり基本を固めておこう。共通テストでは思考力を問う問題が多いから、定義の正確な理解と応用力が勝負の分かれ目になる。
TOEIC
共通試験
理科
英語コミュニケーション
数学
国語
社会Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
90
•
Updated Mar 19, 2026
•
約数と倍数、素数の基本
1 / 6
約数と倍数、素数の基本概念
君たちが普段何気なく使っている「割り切れる」という概念が、実は数学の重要な土台になってるんだ。整数bで整数aが割り切れるとき、つまりa=bkとなる整数kが存在するとき、bはaの約数、aはbの倍数って呼ぶよ。
素数は1とその数自身以外に正の約数を持たない、1より大きい自然数のこと。2, 3, 5, 7, 11...が代表例で、2は唯一の偶数の素数だから覚えておこう。逆に合成数は1より大きい自然数で素数じゃないもの。
ここで絶対に忘れちゃいけないのは、1は素数でも合成数でもないってこと！これは入試でよく引っかけ問題に使われるから要注意だ。
重要ポイント: どんな合成数も素因数分解の形はただ一通りに決まる（素因数の一意性）。この性質が後の計算の根拠になる。
最大公約数と最小公倍数の定義
**最大公約数（GCD）**は2つ以上の整数に共通する約数のうち最も大きいもので、**最小公倍数（LCM）**は共通する正の倍数のうち最も小さいものだ。この2つの概念は入試問題で頻出だから、確実にマスターしよう。
2つの整数の最大公約数が1のとき、それらは互いに素って言う。例えば8と9は互いに素（両方が素数である必要はないよ）。
素因数分解を使ったGCDとLCMの求め方が基本中の基本。まず各数を素因数分解して、GCDは各素因数の指数の最小値、LCMは各素因数の指数の最大値を取る。
暗記必須の公式: 2つの自然数a,bについて、ab = GCD × LCM が常に成り立つ。ただし3つ以上の数では一般に成り立たないから注意！
素因数分解による計算方法
実際の計算例で理解を深めよう。84と90のGCDとLCMを求める場合を見てみる。
まず素因数分解：84 = 2² × 3¹ × 7¹、90 = 2¹ × 3² × 5¹。GCDでは共通する素因数2と3について、指数の小さい方を選んで2¹ × 3¹ = 6。
LCMでは全ての素因数2,3,5,7について、指数の大きい方を選んで2² × 3² × 5¹ × 7¹ = 1260になる。
この方法なら複雑な数でも確実に計算できるし、3つ以上の数でも同じルールが適用できる。どんな問題でもまず素因数分解から考える癖をつけよう。
計算のコツ: 指数を比較するとき、ない素因数は指数0として考える。84には5⁰ = 1があると考えて、5¹と比較する。
ユークリッドの互除法
数が大きくて素因数分解が困難な場合の救世主がユークリッドの互除法だ。この方法は不定方程式の基礎にもなるから絶対マスターしておこう。
原理はシンプル：gcd(a,b) = gcd(b, aをbで割った余り)が成り立つことを利用する。大きい方を小さい方で割って、その余りで次の計算を続け、余りが0になるまで繰り返す。
例：gcd(272,119)の場合、272÷119=2余り34、119÷34=3余り17、34÷17=2余り0。余りが0になったときの直前の余り17が答えだ。
この手順は機械的にできるから、計算ミスが起こりにくい。大きい数の問題では素因数分解より圧倒的に早いよ。
応用への橋渡し: ユークリッドの互除法の計算過程は、不定方程式ax+by=cの特殊解を求めるのに直接使える貴重な情報になる。
実践問題での解法パターン
3つの数54, 72, 180のGCDとLCMを求める問題で実践してみよう。まず素因数分解：54 = 2¹ × 3³、72 = 2³ × 3²、180 = 2² × 3² × 5¹。
GCDは共通する素因数2と3について最小の指数：min(1,3,2)=1、min(3,2,2)=2だから2¹ × 3² = 18。LCMは全ての素因数について最大の指数：max(1,3,2)=3、max(3,2,2)=3、max(0,0,1)=1だから2³ × 3³ × 5¹ = 1080。
「nと24のLCMが360になるnをすべて求めよ」のような逆算問題では、nを n = 2ᵃ × 3ᵇ × 5ᶜ × ... の形で置いて、LCMの定義に従って各指数の条件を求める。
逆算問題のコツ: 求める数の素因数分解の形を文字で置いて、与えられた条件から各指数の値を特定する。系統的にやれば取りこぼしがない。
入試で狙われる重要ポイント
入試でよく引っかかるポイントを整理しておこう。1は素数ではないのは定義の問題で、これを忘れると失点する。互いに素は「共通の約数が1のみ」という意味で、両方が素数である必要はない（4と9は互いに素）。
GCDとLCMを求めるとき、GCDは指数の小さい方、LCMは指数の大きい方を取る。この逆をやってしまうミスが本当に多いから要注意だ。
ab = GCD × LCM の公式は2つの数のときだけ成り立つ。3つ以上では一般に成り立たないから、安易に使わないこと。
「平方数になる」という条件は、素因数分解したときの全ての指数が偶数になる、という条件に言い換えられる。この変換ができると問題が一気に解きやすくなるよ。
最終確認: 素因数分解→GCD/LCM→公式の活用という流れを身につければ、どんな問題でも対応できる。基本を確実にして応用問題に挑もう！
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
Where can I download the Knowunity app?
You can download the app in the Google Play Store and in the Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content in TOEIC
Most popular content
Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5
App Store
4.7/5
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
約数と倍数、素数の基本
整数の性質は大学入試数学Aの超重要分野だよ！約数・倍数・素数の概念は、不定方程式などの他の分野を解く上での基礎になるから、ここでしっかり基本を固めておこう。共通テストでは思考力を問う問題が多いから、定義の正確な理解と応用力が勝負の分かれ目になる。
約数と倍数、素数の基本概念
君たちが普段何気なく使っている「割り切れる」という概念が、実は数学の重要な土台になってるんだ。整数bで整数aが割り切れるとき、つまりa=bkとなる整数kが存在するとき、bはaの約数、aはbの倍数って呼ぶよ。
素数は1とその数自身以外に正の約数を持たない、1より大きい自然数のこと。2, 3, 5, 7, 11...が代表例で、2は唯一の偶数の素数だから覚えておこう。逆に合成数は1より大きい自然数で素数じゃないもの。
ここで絶対に忘れちゃいけないのは、1は素数でも合成数でもないってこと！これは入試でよく引っかけ問題に使われるから要注意だ。
重要ポイント: どんな合成数も素因数分解の形はただ一通りに決まる（素因数の一意性）。この性質が後の計算の根拠になる。
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
最大公約数と最小公倍数の定義
**最大公約数（GCD）**は2つ以上の整数に共通する約数のうち最も大きいもので、**最小公倍数（LCM）**は共通する正の倍数のうち最も小さいものだ。この2つの概念は入試問題で頻出だから、確実にマスターしよう。
2つの整数の最大公約数が1のとき、それらは互いに素って言う。例えば8と9は互いに素（両方が素数である必要はないよ）。
素因数分解を使ったGCDとLCMの求め方が基本中の基本。まず各数を素因数分解して、GCDは各素因数の指数の最小値、LCMは各素因数の指数の最大値を取る。
暗記必須の公式: 2つの自然数a,bについて、ab = GCD × LCM が常に成り立つ。ただし3つ以上の数では一般に成り立たないから注意！
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
素因数分解による計算方法
実際の計算例で理解を深めよう。84と90のGCDとLCMを求める場合を見てみる。
まず素因数分解：84 = 2² × 3¹ × 7¹、90 = 2¹ × 3² × 5¹。GCDでは共通する素因数2と3について、指数の小さい方を選んで2¹ × 3¹ = 6。
LCMでは全ての素因数2,3,5,7について、指数の大きい方を選んで2² × 3² × 5¹ × 7¹ = 1260になる。
この方法なら複雑な数でも確実に計算できるし、3つ以上の数でも同じルールが適用できる。どんな問題でもまず素因数分解から考える癖をつけよう。
計算のコツ: 指数を比較するとき、ない素因数は指数0として考える。84には5⁰ = 1があると考えて、5¹と比較する。
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
ユークリッドの互除法
数が大きくて素因数分解が困難な場合の救世主がユークリッドの互除法だ。この方法は不定方程式の基礎にもなるから絶対マスターしておこう。
原理はシンプル：gcd(a,b) = gcd(b, aをbで割った余り)が成り立つことを利用する。大きい方を小さい方で割って、その余りで次の計算を続け、余りが0になるまで繰り返す。
例：gcd(272,119)の場合、272÷119=2余り34、119÷34=3余り17、34÷17=2余り0。余りが0になったときの直前の余り17が答えだ。
この手順は機械的にできるから、計算ミスが起こりにくい。大きい数の問題では素因数分解より圧倒的に早いよ。
応用への橋渡し: ユークリッドの互除法の計算過程は、不定方程式ax+by=cの特殊解を求めるのに直接使える貴重な情報になる。
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
実践問題での解法パターン
3つの数54, 72, 180のGCDとLCMを求める問題で実践してみよう。まず素因数分解：54 = 2¹ × 3³、72 = 2³ × 3²、180 = 2² × 3² × 5¹。
GCDは共通する素因数2と3について最小の指数：min(1,3,2)=1、min(3,2,2)=2だから2¹ × 3² = 18。LCMは全ての素因数について最大の指数：max(1,3,2)=3、max(3,2,2)=3、max(0,0,1)=1だから2³ × 3³ × 5¹ = 1080。
「nと24のLCMが360になるnをすべて求めよ」のような逆算問題では、nを n = 2ᵃ × 3ᵇ × 5ᶜ × ... の形で置いて、LCMの定義に従って各指数の条件を求める。
逆算問題のコツ: 求める数の素因数分解の形を文字で置いて、与えられた条件から各指数の値を特定する。系統的にやれば取りこぼしがない。
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
入試で狙われる重要ポイント
入試でよく引っかかるポイントを整理しておこう。1は素数ではないのは定義の問題で、これを忘れると失点する。互いに素は「共通の約数が1のみ」という意味で、両方が素数である必要はない（4と9は互いに素）。
GCDとLCMを求めるとき、GCDは指数の小さい方、LCMは指数の大きい方を取る。この逆をやってしまうミスが本当に多いから要注意だ。
ab = GCD × LCM の公式は2つの数のときだけ成り立つ。3つ以上では一般に成り立たないから、安易に使わないこと。
「平方数になる」という条件は、素因数分解したときの全ての指数が偶数になる、という条件に言い換えられる。この変換ができると問題が一気に解きやすくなるよ。
最終確認: 素因数分解→GCD/LCM→公式の活用という流れを身につければ、どんな問題でも対応できる。基本を確実にして応用問題に挑もう！
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
Where can I download the Knowunity app?
You can download the app in the Google Play Store and in the Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
0
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Practice Test ✓ Essay Outlines
Practice Test
Quiz
Flashcards
Essay
Most popular content in TOEIC
Most popular content
Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5
App Store
4.7/5
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user