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Updated Mar 27, 2026
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Comprendre le Raisonnement par Récurrence
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Les bases du raisonnement par récurrence
Tu vas découvrir une technique de démonstration géniale qui fonctionne en deux étapes simples. Le principe de récurrence te permet de prouver des formules mathématiques pour tous les nombres entiers d'un coup !
Imagine que tu veux prouver qu'une propriété P(n) est vraie pour tous les entiers n ≥ 1. Il suffit de montrer deux choses : d'abord que P(1) est vraie (initialisation), puis que si P(k) est vraie, alors P l'est aussi (hérédité).
💡 Astuce : C'est exactement comme les dominos qui tombent - si le premier tombe et que chaque domino fait tomber le suivant, alors tous tombent !
Exercice 1 : La somme des nombres impairs
Regarde ces égalités surprenantes : 1 = 1², 1+3 = 2², 1+3+5 = 3²... Tu vois le motif ? La somme des n premiers nombres impairs égale toujours n² !
Pour le prouver par récurrence, on vérifie d'abord que c'est vrai pour n=1 (1 = 1²). Ensuite, on suppose que c'est vrai pour k et on montre que ça marche pour k+1.
L'astuce géniale : la somme des premiers nombres impairs = (somme des k premiers) + (le nombre impair suivant). Avec l'hypothèse de récurrence, ça donne k² + = ² !
💡 À retenir : Le n-ième nombre impair s'écrit toujours 2n-1.
Exercice 2 : L'inégalité de Bernoulli
L'inégalité de Bernoulli dit que ⁿ ≥ 1+nx quand x > -1 et n ≥ 0. Pourquoi cette condition x > -1 ? Parce qu'on va multiplier par pendant la démonstration !
Si x ≤ -1, alors 1+x ≤ 0. Multiplier une inégalité par un nombre négatif change son sens (≥ devient ≤). Avec x > -1, on garde 1+x > 0, donc l'inégalité garde son sens.
La preuve par récurrence fonctionne parfaitement : on multiplie l'hypothèse ᵏ ≥ 1+kx par , puis on utilise le fait que kx² ≥ 0 pour obtenir le résultat !
💡 Piège à éviter : Ne jamais oublier de vérifier les conditions avant de multiplier une inégalité !
Exercice 3 : Suite et divisibilité
Avec la suite récurrente u₀ = 7 et uₙ₊₁ = 3uₙ - 2, tu calcules facilement : u₁ = 19, u₂ = 55. Maintenant observe uₙ - 1 : pour u₀, c'est 6 ; pour u₁, c'est 18 ; pour u₂, c'est 54.
Tous ces nombres sont divisibles par 6 ! C'est notre conjecture à prouver par récurrence. L'initialisation est évidente avec u₀ - 1 = 6.
Pour l'hérédité, l'astuce est de calculer uₖ₊₁ - 1 = 3uₖ - 2 - 1 = 3uₖ - 3 = 3. Si uₖ - 1 est divisible par 6, alors uₖ₊₁ - 1 = 3 l'est aussi !
💡 Technique : Exprime toujours le terme suivant en fonction du terme précédent pour utiliser l'hypothèse de récurrence.
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Most popular content: Raisonnement mathématique
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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
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
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
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
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 apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
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
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
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
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
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 apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
Comprendre le Raisonnement par Récurrence
Le raisonnement par récurrenceest une méthode super utile en maths pour prouver qu'une propriété est vraie pour tous les nombres entiers. C'est comme monter un escalier infini : on vérifie qu'on peut monter la première marche, puis qu'on peut... Show more
Les bases du raisonnement par récurrence
Tu vas découvrir une technique de démonstration géniale qui fonctionne en deux étapes simples. Le principe de récurrence te permet de prouver des formules mathématiques pour tous les nombres entiers d'un coup !
Imagine que tu veux prouver qu'une propriété P(n) est vraie pour tous les entiers n ≥ 1. Il suffit de montrer deux choses : d'abord que P(1) est vraie (initialisation), puis que si P(k) est vraie, alors P l'est aussi (hérédité).
💡 Astuce : C'est exactement comme les dominos qui tombent - si le premier tombe et que chaque domino fait tomber le suivant, alors tous tombent !
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
Exercice 1 : La somme des nombres impairs
Regarde ces égalités surprenantes : 1 = 1², 1+3 = 2², 1+3+5 = 3²... Tu vois le motif ? La somme des n premiers nombres impairs égale toujours n² !
Pour le prouver par récurrence, on vérifie d'abord que c'est vrai pour n=1 (1 = 1²). Ensuite, on suppose que c'est vrai pour k et on montre que ça marche pour k+1.
L'astuce géniale : la somme des premiers nombres impairs = (somme des k premiers) + (le nombre impair suivant). Avec l'hypothèse de récurrence, ça donne k² + = ² !
💡 À retenir : Le n-ième nombre impair s'écrit toujours 2n-1.
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
Exercice 2 : L'inégalité de Bernoulli
L'inégalité de Bernoulli dit que ⁿ ≥ 1+nx quand x > -1 et n ≥ 0. Pourquoi cette condition x > -1 ? Parce qu'on va multiplier par pendant la démonstration !
Si x ≤ -1, alors 1+x ≤ 0. Multiplier une inégalité par un nombre négatif change son sens (≥ devient ≤). Avec x > -1, on garde 1+x > 0, donc l'inégalité garde son sens.
La preuve par récurrence fonctionne parfaitement : on multiplie l'hypothèse ᵏ ≥ 1+kx par , puis on utilise le fait que kx² ≥ 0 pour obtenir le résultat !
💡 Piège à éviter : Ne jamais oublier de vérifier les conditions avant de multiplier une inégalité !
Sign up to see the contentIt's free!
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Exercice 3 : Suite et divisibilité
Avec la suite récurrente u₀ = 7 et uₙ₊₁ = 3uₙ - 2, tu calcules facilement : u₁ = 19, u₂ = 55. Maintenant observe uₙ - 1 : pour u₀, c'est 6 ; pour u₁, c'est 18 ; pour u₂, c'est 54.
Tous ces nombres sont divisibles par 6 ! C'est notre conjecture à prouver par récurrence. L'initialisation est évidente avec u₀ - 1 = 6.
Pour l'hérédité, l'astuce est de calculer uₖ₊₁ - 1 = 3uₖ - 2 - 1 = 3uₖ - 3 = 3. Si uₖ - 1 est divisible par 6, alors uₖ₊₁ - 1 = 3 l'est aussi !
💡 Technique : Exprime toujours le terme suivant en fonction du terme précédent pour utiliser l'hypothèse de récurrence.
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and 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.
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Most popular content: Raisonnement mathématique
Mathématiques : Suites arithmétiques et géométriques exercices + contrôle corrigé
Suites arithmétiques et géométriques : le exercices + contrôle corrigé
MathsTle
Most popular content in Maths
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
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
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
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
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 apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
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
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
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
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
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 apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user