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I have the following equation :

$2 \sin(3a)=\sqrt{2}$

Not sure how to solve it (Because it's a transformed sin function, meaning 6 solution with 3 cycles in $2\pi$) after a moment I finally found that each solution can be expressed as a multiple of $\frac{\pi}{12}$ (I aided myself with a graph of the function.)

For example :

$\frac{\pi}{12} \frac{3\pi}{12} \frac{9\pi}{12}$ etc.

My question would be : Is this the standard way of solving transformed functions of this sort ? Is there a sort of symbolic way without the need to use the graph ?

Is my way complicated for nothing ? Thank you !

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1  
What happens when you take $\arcsin$ of both sides? –  Igor Rivin Dec 6 '13 at 22:50
    
Well when I take the arcsin of sqrt(2)/2 I get pi/4, but my calculator considers it like a normal sin function without any transformation –  Astroman Dec 6 '13 at 22:54

3 Answers 3

Hints:

$$2\sin3a=\sqrt2\iff \sin 3a=\frac{\sqrt2}2=\frac1{\sqrt2}\iff 3a=\begin{cases}\frac\pi4\\{}\\\frac{3\pi}4\end{cases}+2k\pi\;,\;k\in\Bbb Z\;\ldots$$

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why did you put it in the form 1/sqrt(2) –  Astroman Dec 6 '13 at 23:19
    
Habit, @Astroman: it's the same, of course, and it's one of the basic values of basic angles for sine and cosine. –  DonAntonio Dec 6 '13 at 23:23
1  
This is the best answer +1 –  user66360 Dec 6 '13 at 23:54

$2 \sin(3a)=\sqrt{2}$

$\implies \sin(3a)=\frac{\sqrt{2}}{2}$

$\implies \arcsin(\sin(3a))=\arcsin\left({\frac{\sqrt{2}}{2}}\right)$

$\implies 3a=\arcsin\left({\frac{\sqrt{2}}{2}}\right)$

Does that help?

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Well I did find pi/12 (Which I got in a geometrical way) –  Astroman Dec 6 '13 at 22:59
    
First of all, let's assume $0\le a \le 2\pi$. Then I'd just look at the unit circle. You see two places in which $y=\frac{\sqrt{2}}{2}$: namely at $\frac{\pi}{4}$ and $\frac{3\pi}{4}.$ Hence, $\arcsin\left({\frac{\sqrt{2}}{2}}\right)=$$\frac{\pi}{4}$ and $\frac{3\pi}{4}$. If you divide by 3, you have $\frac{\pi}{12}$ and $\frac{3\pi}{12}.$ –  user66360 Dec 6 '13 at 23:05
    
ok.... and after ? –  Astroman Dec 6 '13 at 23:18
    
$a=\frac{\pi}{12}$ and $\frac{3\pi}{12}$. Was the explanation unclear? –  user66360 Dec 6 '13 at 23:21
    
No, I understand but I'm not sure how to have the other 4 solutions ... –  Astroman Dec 6 '13 at 23:22

Hint:

$$\sin x = y \iff x = \arcsin y + 2\,k\,\pi \;\;\mathrm{or}\;\; x = \pi - \arcsin y + 2\,k\,\pi $$

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