•

Updated Apr 5, 2026

•

1 page

弧度法と角の測り方

三角関数って難しそうに見えるけど、実は円の上の点の動きを表しているだけなんだ。今回は角度の新しい表し方「弧度法」と、それを使った三角関数の基本をマスターしよう。

$# 一般角と弧度法 ## 1. 弧度法 * **弧度法** 半径$r$の円において、長さ$r$の弧に対する中心角の大きさを1ラジアンといい、角の大きさを弧の長さで表す方法を弧度法という。このとき、半径$r$の円において、長さ$l$の弧に対する中心角の大きさ$\th$

一般角と弧度法

角度を表すのに「度」以外の方法があるって知ってた？弧度法（ラジアン） は、円の弧の長さを使って角度を表す超実用的な方法なんだ。

弧度法の仕組みは簡単で、半径rの円で長さlの弧があるとき、中心角θ = l/r で計算できる。つまり半径と同じ長さの弧ができる角度が1ラジアンってことだ。

度数法との変換で覚えておくべきは180° = π rad。これさえ覚えれば、30°なら π/6、90°なら π/2 みたいに計算できるよ。

扇形の計算も簡単になる。弧の長さl = rθ、面積S = (1/2)rl で一発だ。度数法より計算がスッキリするから、慣れると弧度法の方が楽になる。

💡 覚え方のコツ: π = 180°を基準に、よく使う角度（30°、45°、60°、90°）の弧度法での表し方を丸暗記しちゃおう！

三角関数の定義も単位円（半径1の円）で考えると分かりやすい。点P(x,y)があるとき、sinθ = y/r、cosθ = x/r、tanθ = y/x だ。

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.

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

Stefan S

iOS user

Samantha Klich

Android user

Anna

iOS user

Best app on earth! no words because it’s too good

Thomas R

iOS user

Basil

Android user

David K

iOS user

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

Rohan U

Android user

Xander S

iOS user

Elisha

iOS user

Paul T

iOS user

算数

•

Updated Apr 5, 2026

•

1 page

弧度法と角の測り方

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

一般角と弧度法

角度を表すのに「度」以外の方法があるって知ってた？弧度法（ラジアン） は、円の弧の長さを使って角度を表す超実用的な方法なんだ。

度数法との変換で覚えておくべきは180° = π rad。これさえ覚えれば、30°なら π/6、90°なら π/2 みたいに計算できるよ。

扇形の計算も簡単になる。弧の長さl = rθ、面積S = (1/2)rl で一発だ。度数法より計算がスッキリするから、慣れると弧度法の方が楽になる。

💡 覚え方のコツ: π = 180°を基準に、よく使う角度（30°、45°、60°、90°）の弧度法での表し方を丸暗記しちゃおう！

三角関数の定義も単位円（半径1の円）で考えると分かりやすい。点P(x,y)があるとき、sinθ = y/r、cosθ = x/r、tanθ = y/x だ。