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How can I prove that $\sin (10^\circ), \sin(1^\circ), \sin(2^\circ), \sin(3^\circ), \tan(10^\circ)$ are irrational?

My try:: Let $x = 10^\circ$, Then $3x = 30^\circ$

Now $\sin (3x) = \sin (30^\circ)$

So $\displaystyle 3\sin (10^\circ)-4\sin^3(0^\circ) = \frac{1}{2}$

Let $x = \sin (10^\circ)$

Then $\displaystyle 3x-4x^3 = \frac{1}{2}\Leftrightarrow 6x-8x^3=1$

$8x^3-6x+1 = 0$

Now How Can i calculate Roots of Given equation.

plz explain me

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You don't need to find the roots, you just need to show that they aren't rational. –  Git Gud May 19 '13 at 16:00
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I'm assuming you mean, e.g. $\sin(10^\circ)$ and not $\sin(10^0)$? –  amWhy May 19 '13 at 16:01
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Thanks Git Gud but how can i prove that they are not rational –  juantheron May 19 '13 at 16:02
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to put degree sign on angle use ^\circ between $ sign –  iostream007 May 19 '13 at 16:02
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3 Answers 3

up vote 2 down vote accepted

Here is a different approach for $\sin(1^\circ)=\sin\left(\frac{2\pi}{360}\right)$ (the other cases are similar).

The complex numbers $\zeta_{1,2}=e^{\pm\frac{2\pi}{360}i}$ are algebraic integers (are roots of $x^{360}-1$) and therefore $\zeta_1+\zeta_2=2\sin\left(\frac{2\pi}{360}\right)$ is algebraic integer. If $\sin\left(\frac{2\pi}{360}\right)$ is rational then $2\sin\left(\frac{2\pi}{360}\right)$ is rational and therefore integer (the only rational algebraic integers are the integers).
So $2\sin(1^\circ)=-2,-1,0,1$ or $2 \Rightarrow\Leftarrow$.

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Note: All that you want to show is that it's not rational.

Hint: Apply the rational root theorem. What possible rational roots are there? Now check if those are indeed roots.

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One can use a feature of complex numbers, and the span of a finite set.

Consider the set of cyclotomic numbers, ie $C(n) = cis(2\pi/n)^m$, where $cis(x)=cos(x)+i\cdot(sin(x)$. Such a set is closed to multiplication. The 'Z-span' of the set is the set of values $sum(a_m cis(2\pi/n)$, over n, is also closed to multiplication.

We now begin with the observation that a span of a finite set, closed to multiplication, can not include the fractions. This is proved by showing that if a rational number, not an integer, is in the set, so must all of its powers. (ie if 1/2 is constructable by steps at multiples of N°, (eg a random walk of unit-size steps at exact degrees), so must all values of $1/2^a$).

Since this means that that the intersection of the cyclotomic numbers $\mathbb{C}_n$ and the rationals $\mathbb{F}$ can not include any fractions, and thus must give $\mathbb{Z}$.

The double-cosine of the half-angles, are given by $1-cis(2\pi/n)$, and therefore we see that the only rational numbers that can occur in the sines and cosines, is 1/2. The chord, and the supplement-chords are entirely free of rationals, and further, no product of such numbers can be rational.

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