Tuesday, 2 July 2013

trigonometry - Solving $sqrt{3} sin(phi-pi/6)=sin(phi)$



The question is to solve:
$$ \sqrt{3} \sin \left ( \phi - \frac \pi 6 \right )= \sin \phi $$




I tried to turn it into $a \sin \phi-b \cos\phi= \sin\phi$,



Then I got $ \frac 32 \sin\phi - \frac 3 4 \cos \phi= \sin\phi$,



Therefore, $ \frac 12 \sin\phi- \frac 3 4 \cos\phi=0$.



Then I turned it back into $R\sin(\phi-a)=0$,



$\implies (√13/4)\sin(ø-0.983)=0$




So I think the solution should be $\phi=n\pi+0.983$



Is that correct? The answer on my book is $n\pi+\frac \pi 3$.(I made a mistake here, It should be π/3.)



Update: The reason why I got the wrong answer is because I forgot to √ that 4/3.



Thanks for pointing that mistake out :D.


Answer



It may be directly written that:

$$\sqrt3 \cos\phi= \sin \phi \implies \tan\phi = \sqrt{3} \implies \phi = n \pi + \dfrac{\pi}3 \text{ where }n\in \mathbb{Z}$$



Q.E.D.


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