MHB Trigonometry Challenge II: Solving for $m,\,n$ and $A$

AI Thread Summary
The discussion focuses on solving the equation $\sqrt{9-8\cos 40^{\circ}}=m+n\cos A^{\circ}$ for natural numbers $m$ and $n$, and angle $A$. The solution reveals that by manipulating trigonometric identities, the expression simplifies to $(1 + 4\sin 10^{\circ})$, which can also be expressed as $(1 + 4\cos 80^{\circ})$. The values obtained are $m=1$, $n=4$, and $A=80$. Participants share their methods and calculations, confirming the correctness of the solution. The thread emphasizes the use of trigonometric identities to derive the final results.
anemone
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Find $m,\,n$ and $A$ such that $\sqrt{9-8\cos 40^{\circ}}=m+n\cos A^{\circ}$ where $m,\,n\in N$.
 
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anemone said:
Find $m,\,n$ and $A$ such that $\sqrt{9-8\cos 40^{\circ}}=m+n\cos A^{\circ}$ where $m,\,n\in N$.
I sort of cheated here by using a calculator to guess the answer, and then working backwards.

[sp]The formula $\sin3\theta = 3\sin\theta - 4\sin^3\theta$ shows that $\frac12= \sin30^\circ = 3\sin10^\circ - 4\sin^310^\circ$, so that $8\sin^310^\circ = 6\sin10^\circ - 1$. Then $$\begin{aligned} 9 - 8\cos40^\circ &= 9 - 8(1-2\sin^220^\circ) \\ &= 1 + 16\sin^220^\circ \\ &= 1 + 16(2\sin10^\circ\cos10^\circ)^2 \\ &= 1 + 64\sin^210^\circ(1-\sin^210^\circ) \\&= 1 + 64\sin^210^\circ - 64\sin^410^\circ \\&= 1 + 64\sin^210^\circ - 8\sin10^\circ(6\sin10^\circ - 1) \\ &= 1 + 8\sin10^\circ + 16\sin^210^\circ \\ &= (1+4\sin10^\circ)^2.\end{aligned}$$ Therefore $\sqrt{\mathstrut9 - 8\cos40^\circ} = 1+4\sin10^\circ = 1 + 4\cos80^\circ$. So $m=1,\ n=4,\ A=80$.[/sp]
 
Hi Opalg,

Thanks for your solution and thanks too for being honest to me. :)

A quite similar solution (of other) that I want to share with MHB:

$\begin{align*}\sqrt{9-8\cos 40^{\circ}}&=\sqrt{9-8(2\sin 30^{\circ}\cos 40^{\circ})}\\&=\sqrt{9-8(\sin 70^{\circ}-\sin 10^{\circ})}\\&=\sqrt{9-8(1-2\sin^2 10^{\circ}-\sin 10^{\circ} )}\\&=\sqrt{1+8\sin 10^{\circ}+16\sin^2 10^{\circ}}\\&=\sqrt{(1+4\sin 10^{\circ})^2}\\&=1+4\sin 10^{\circ}\\&=1+4\cos 80^{\circ}\end{align*}$

Therefore, $m=1,\,n=4$ and $A=80$.
 
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