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In the book 101 problems in Trigonometry, Prof. Titu Andreescu and Prof. Feng asks for the proof the fact that $\cos 1^\circ$ is irrational and he proves it. The proof proceeds by contradiction and using the strong induction principle. (Problem on Pg:84, 70 in typeset; solution on Pg:126, 111 in Typeset). However, for Completeness, I'll append it here:

Proof of irrationality of $\cos (1^\circ)$

Assume for the sake of contradiction, that $\cos(1^\circ)$ is rational. Since, $$\cos(2^\circ)=2\cos^2(1^\circ)-1$$ we have that, $\cos(2^\circ)$ is also rational. Note that, we also have $$\cos(n^\circ +1 ^\circ)+\cos(n^\circ -1 ^\circ)=2\cos(1^\circ)\cdot \cos(n^\circ)$$ By Strong induction principle, this shows that $\cos(n^\circ)$ is rational for all integers $n \geq 1$. But, this is clearly false, as for instance, $\cos(30^\circ)=\dfrac{\sqrt{3}}{2}$ is irrational, reaching a contradiction.

But, as my title suggests, $\sin(1^\circ)$ is irrational, (look at the following image for its value!) Sine 1 degree

Is there a proof as short as the above proof or can any of you help me with a proof that bypasses actual evaluation of the above value?

Image Courtesy: http://www.efnet-math.org/Meta/sine1.htm This link explains how to evaluate this value.

My next question is

Is $\tan(1^\circ)$ rational and is there a short proof that asserts or refutes its rationality?

P.S.: This is not a homework question.

  • 7
    We seen a related question here recently: For which rational values of $\theta^\circ$ is $\sin\theta^\circ$ rational? And the answer turned out to be: _only_ the ones you learned at your mother's knee.2011-12-27
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    @Hardy: You could consider elaborating your cryptic. I have been trying to get hold of the link to the question without any luck.2011-12-27
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    @HenningMakholm:Brain glitch-deleted. You are correct2011-12-27
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    [This](http://math.stackexchange.com/questions/15823/sine-values-being-rational) question, taken literally, doesn't ask the same thing, but some of the answers, including the accepted one, seemed to construe it that way.2011-12-27
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    @Hardy Thanks for the pointer. The magic word is Niven's Theorem. Also, I'm planning to read upon the CMJ article you have referred to in your Wiki Article.2011-12-27
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    Converted to CW on OP's request.2012-05-08
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    After years ... this problem reappears on the Princeton University Math Competition. See **[here](http://www.artofproblemsolving.com/Forum/viewtopic.php?f=414&t=563991)**.2013-11-24
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    In a very slick manner, you can use the [Chebyshev polynomials of the first kind](https://en.wikipedia.org/wiki/Chebyshev_polynomials#Trigonometric_definition) with the rational roots theorem to deduce $\cos(\frac{90}x)$ is rational or irrational.2016-11-12

7 Answers 7

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We have $\sin(5*x)=16*\sin(x)^5-20*\sin(x)^3+5*\sin(x)$ and $\sin(3*x)=3*\sin(x)-4*\sin(x)^3$

Suppose that $\sin(1)$ is rational then according to first relation $\sin(5)$ is rational.Now by using relation $2$ conclude that $\sin(15)$ is rational. repeat this process and by using again of relation two conclude that $\sin(45)$ is rational. We know that it is the half of the root of $2$ and this is a contradiction so $\sin(1)$ is irrational

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    Welcome to MSE. For some basic information about writing mathematics at this site see, *e.g.*, [basic help on mathjax notation](/help/notation), [mathjax tutorial and quick reference](//math.meta.stackexchange.com/q/5020), [main meta site math tutorial](//meta.stackexchange.com/a/70559) and [equation editing how-to](//math.meta.stackexchange.com/q/1773).2018-07-01
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If it's a slick proof you want, nothing beats this proof that the only cases where both $r$ and $\cos(r \pi)$ are rational are where $\cos(r \pi)$ is $-1$, $-1/2$, $0$, $1/2$ or $1$.

If $r=m/n$ is rational, $e^{i \pi r}$ and $e^{-i \pi r}$ are roots of $z^{2n} - 1$, so they are algebraic integers. Therefore $2 \cos(r \pi) = e^{i \pi r} + e^{-i \pi r}$ is an algebraic integer. But the only algebraic integers that are rational numbers are the ordinary integers. So $2 \cos(r \pi)$ must be an integer, and of course the only integers in the interval $[-2,2]$ are $-2,-1,0,1,2$.

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    I loved your answer. But, I am choosing @Henning's answer just because it also tells me how to handle this $\tan(1^\circ)$. Yours I think is called Niven's Theorem and I just like it. So, No offence.2011-12-28
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    I thank @Srivatsan for helping me understand your solution through the chat2011-12-28
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    Robert isreal- may you explain more about how come the algebraic integer and ordinary integer is involved here.2012-01-28
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$\sin(1^\circ) = \cos(89^\circ)$, and since 89 is relatively prime to 360, the proof for $\cos 1^\circ$ works with almost no change.

More precisely: Assume that $\cos(89^\circ)$ is rational. Then, by the same induction as before with every $1^\circ$ replaced by $89^\circ$ we get that $\cos(89n^\circ)$ is rational for every $n\in\mathbb N$. In particular, since $150\times 89=37\times 360+30$, we get that $$\cos(150\times 89^\circ)=\cos(37\times 360^\circ+30^\circ)=\cos(30^\circ)$$ is rational, a contradiction.

For $\tan(1^\circ)$, a slight variant of the same proof works. Assume that $\alpha = \tan(1^\circ)$ is rational. Then $1+\alpha i$ is in $\mathbb Q[i]$, and then $\tan(n^\circ)$, being the ratio between the imaginary and real parts of $(1+\alpha i)^n$ is also rational. But $\tan(30^\circ)$ is not rational, so $\tan(1^\circ)$ cannot be either.

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    I don't quite understand how this co-primeness comes into play. And, I also am not sure if this proof will go through. I assume that $\cos(89 ^\circ)$ is rational. Then, I can prove that $\cos^2(2^\circ)$ is rational, hence $\cos(4^\circ)$ ir rational.This by induction prove $\cos(30^\circ)$ is rational, a contradiction.2011-12-27
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    I can't quite follow the second step in your sketch, but I've edited my answer to show more explicitly what I had mind mind.2011-12-27
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    +1 I got the answer finally, with ample help from @DylanMoreland2011-12-27
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You can prove it exactly the same way:

Assume by contradiction that $\sin(1^\circ)$ is rational.

Then

$$\cos(2^\circ)=1- 2\sin^2(1^\circ) \mbox{is rational}\,,$$

Now you can also prove that $\cos 4^\circ$ is rational.

Using

$$\cos((2n+2)^\circ)+\cos((2n-2) ^\circ)=2\cos(2^\circ)\cdot \cos((2n)^\circ) \,,$$

you can prove by induction that $\cos(2n^\circ)$ is rational, and you get your contradiction...

Added

If $\tan(1^\circ)$ is rational, then

$$\cos(2^\circ) =\frac{1- \tan^2(1^\circ)}{1+\tan^2(1^\circ)}$$ is also rational...

Alternately, if you are not familiar with this relation, note that

$$\cos^2(1^\circ)= \frac{1}{\sec^2(1^\circ)}= \frac{1}{1+ \tan^2(1^\circ)}$$ is rational, thus

$$\cos(2^\circ)=2\cos^2(1^\circ)-1$$ is rational.

The first part of the proof finishes this part too...

P.S. You can actually prove by induction the following result: if $\cos(x)$ is rational, then $\cos(nx)$ is also rational.

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    Don't we also have that $\cos(2^\circ)$ is rational? And If so, it can as well serve as the basis of induction. And why is the second equation required? Please clarify.2011-12-27
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    Now that you have added something new, I am talking about part 1 in my previous comment.2011-12-27
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    yea, I meant $\cos(2^\circ)$ is rational: ) WIll edit it now.2011-12-27
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    Actually this answer will be more helpful to my students.2011-12-28
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Interestingly enough,I happened to encounter this very problem (regarding $\tan 1^\circ$) yesterday.

Let's assume $\tan 1^\circ$ is rational.Then we can repeatedly use the formula $\tan (A+B)=\frac{\tan A+ \tan B}{1- \tan A \tan B}$ to get that $\tan 60^\circ = \sqrt{3}$ is rational as well, a contradiction.

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(Too long for a comment.)

Jack Calcut's article gives the following result:

Corollary: The only rational values of $\tan\,k\pi/n$ are $0$ and $\pm1$.

which rests on the lemma

Main Lemma: Let $z\neq0$ be a Gaussian integer. There is a natural number $n$ such that $z^n$ is real iff either $\Re z=0$, $\Im z=0$, $\Re z=\Im z$, or $\Re z=-\Im z$.

If $\tan\dfrac{k\pi}{n}=\dfrac{q}{p}$, then $\dfrac{k\pi}{n}=\arg(p+iq)$, and $k\pi=\arg((p+iq)^n)$, which implies $(p+iq)^n$ is an integer. If $(p+iq)^n$ is an integer, then $\dfrac{k\pi}{n}$ is an integer multiple of $\pi/4$ by the Main Lemma. Since $\tan$ is $\pi$-periodic, $\tan\,0=0$, and $\tan\dfrac{\pm \pi}{4}=\pm 1$, the corollary is established.


For giggles, I asked Mathematica for the following explicit radical representations:

$$\begin{split}\sin\frac{\pi}{180}=-\frac1{\sqrt[3]{2}}\left(\frac1{32}+\frac{i}{32}\right)&\left(\sqrt[3]{-1-i\sqrt{3}} \left(1-i\sqrt{3}\right)\left(\sqrt{2}+\sqrt{10}-2i\sqrt{5-\sqrt{5}}\right)+\right.\\&\left.\sqrt[3]{-1+i\sqrt{3}}\left(\sqrt{3}-i\right)\left(\sqrt{2}+\sqrt{10}+2i\sqrt{5-\sqrt{5}}\right)\right)\end{split}$$

$$\scriptsize \tan\frac{\pi}{180}=-\frac{\sqrt[3]{-1-i \sqrt{3}} \left(1-i \sqrt{3}\right) \left(\sqrt{2}+\sqrt{10}-2i\sqrt{5-\sqrt{5}}\right)+\sqrt[3]{-1+i\sqrt{3}} \left(\sqrt{3}-i\right)\left(\sqrt{2}+\sqrt{10}+2i\sqrt{5-\sqrt{5}}\right)}{\sqrt[3]{-1-i\sqrt{3}}\left(\sqrt{3}+i\right) \left(\sqrt{2}+\sqrt{10}-2i\sqrt{5-\sqrt{5}}\right)+\sqrt[3]{-1+i\sqrt{3}}\left(\sqrt{3}-i\right) \left(2 \sqrt{5-\sqrt{5}}-i\left(\sqrt{2}+\sqrt{10}\right)\right)}$$

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    @JM I read through the article and the main results it proves. Of particular interest is the reference in the paper, on the irrationality of trigonometric ratios and a subsequent note by Underwood. But, I am sorry for being unable to choose this as an answer. Thanks a bunch for your answer.2011-12-28
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    I intended this as a comment, so it's perfectly fine and no apologies are needed. I also think Henning's answer is a good one to accept.2011-12-28
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Note that $\cos(2x) = \frac{1-\tan^2(x)}{1+\tan^2(x)}$. So if $\tan(x)$ is rational, or even the square root of a rational, $\cos(2x)$ must be rational. Combine this with Niven's Theorem to answer the question about $\tan$.

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    An induction argument again....But, Thank you for taking time.2011-12-28
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    Induction? Where?2011-12-30
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    Oh Sorry, read it carefully again. Now I don't see an induction at work, I am unable to say why I wrote such a comment that day.2011-12-31