# Turn The Pencil Around!

[Warning: This post is quite basic, I mean really basic!]

In an earlier post, we saw a proof without words. On this post we'll see another proof without words. I really loved this when I saw it for the first time and I hope you do too.

Today's topic is really simple: the sum of the angles of a triangle.

We all know that the sum of the angles of any triangle is equal to $180^{\circ}$. We've all learned how to prove that in school.

Some of you probably tore off the corners of a triangle to verify that in some point in your life [if you haven't done this, try it now!].

Alt text

But on this post, we're going to do it in a different way, a way that [hopefully] makes us appreciate the beauty in the simplest of things. Let's get on with it!

We're going to start with a question? What does it mean when you rotate some thing by $180^{\circ}$? If we had a pencil lying horizontally and if we rotated it by $180^\circ$, what would happen?

Alt text

The picture above demonstrates what a rotation by $180^\circ$ looks like. Keep that in mind.

We're going to start by drawing a triangle and putting a pencil horizontally below it.

Alt text

We have to prove that $\angle CBA+ \angle BAC + \angle ACB=180^\circ$.

We're going to start by rotating the pencil by $\angle CBA$.

Alt text

Then by $\angle BAC$...

Alt text

And finally, by $\angle ACB$...

Alt text

Now if you compare how the pencil was before rotation and what it looks like after being rotated by the angles of triangle $ABC$, you'll see that the pencil has rotated exactly by $180^\circ$.

So, $\angle CBA+ \angle BAC + \angle ACB=180^\circ$ [Proved without words!].

This just goes to show that you can look at the simplest of things from a different perspective and enjoy the beauty in them.

Now using the same principle, prove to yourself that the sum of the internal angles of a quadrilateral equals $360^\circ$. Turn the pencil around!

Note by Mursalin Habib
6 years, 7 months ago

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Got something to teach my younger brother thanks for such an interesting post..

- 4 years, 2 months ago

You're welcome! I'm glad you liked it.

- 4 years, 2 months ago

:)

- 6 years, 3 months ago

This is very basic. Even a class 5 student can understand this. Whoever came up with this idea is great :)

- 6 years, 3 months ago

Nice one....

- 6 years, 3 months ago

Simply Fantastic proof!

- 6 years, 6 months ago

Nice! I liked how this proof helped encourage visualization! :)

- 6 years, 7 months ago

wow!! interesting n so simple. i like it.

- 6 years, 7 months ago

Brilliant.

- 6 years, 7 months ago

WOW!

- 6 years, 7 months ago

Mursalin, I think what really needs to be noted is your explanation (using words). I have heard in my math class (I teach a grade 7 and 8 class, this is my first year) "why do we needs to use words?". I think the key here is the instructions you give on how to perform the proof. This is a great proof and there is plenty for any beginning student or new teacher (thanks I will be using this, giving you the credit of course) to take away from "basic" proofs that students can feel confident about presenting. What I would like to see is if you could decompose your proof to only the most essential parts and still consider it a proof. Well done all around!

- 6 years, 7 months ago

Very creative approach. Simply amazing!

- 6 years, 7 months ago

Very interesting read! An elegant proof without words, wonderful.

- 6 years, 7 months ago

Nice!

- 6 years, 7 months ago

সুন্দর :)

- 6 years, 7 months ago

Very interesting Mursalin. And I think this is not basic at all. It is such a creative way of proving!

- 6 years, 7 months ago

Very well, Mursalin! Good work!

- 6 years, 7 months ago

That was a great idea! To quote Feynman- "There are infinite ways in the universe. We just have to hunt them."

- 6 years, 7 months ago

This opened my eyes O.O Interesting.

- 6 years, 7 months ago

Cool!

- 6 years, 7 months ago

A creative new solution is always a enjoyment. It's good to note in the moves of the pencil is that every angle is counter-clockwise in other words, positive by convention. Greetings from Chile.

- 6 years, 7 months ago

I'm glad you enjoyed it!

- 6 years, 7 months ago

I hope this is not too basic, even for #CoSinesGroup.

- 6 years, 7 months ago

Elegantly done.

- 6 years, 3 months ago

Elegant.

- 6 years, 7 months ago

This is very well done, if not just adorable. Math is never "too basic" if you present it well and in an interesting way, as you have done here. Good job!

- 6 years, 7 months ago

Thank you so much for your kind words of encouragement! It really means a lot.

- 6 years, 7 months ago