Melody

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Melody  11 feb 2022
 #3
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+1

Thanks Heureka,

 

Here is another approach.    laugh

 

\(\sin \frac{\pi}{12} + \sin \frac{3\pi}{12} + \sin \frac{5\pi}{12} + \sin \frac{7\pi}{12} + \sin \frac{9\pi}{12} + \sin \frac{11\pi}{12}\\ =\sin \frac{\pi}{12} + \sin \frac{3\pi}{12} + \sin \frac{5\pi}{12} + \sin \frac{5\pi}{12} + \sin \frac{3\pi}{12} + \sin \frac{\pi}{12}\\ =2(\sin \frac{\pi}{12} + \sin \frac{3\pi}{12} + \sin \frac{5\pi}{12} )\\ =2(\sin \frac{\pi}{12} + \sin \frac{\pi}{4} + \cos \frac{\pi}{12} )\\ =2(\sin \frac{\pi}{12} + \cos \frac{\pi}{12} )+2\sin \frac{\pi}{4}\\ =2\sqrt{(\sin \frac{\pi}{12} + \cos \frac{\pi}{12} )^2}+ \frac{2}{\sqrt{2}}\\ =2\sqrt{(\sin^2 \frac{\pi}{12} + \cos^2 \frac{\pi}{12} +2\sin \frac{\pi}{12}\cos \frac{\pi}{12} )}+ \frac{2}{\sqrt{2}}\\ =2\sqrt{(1 +2\sin \frac{\pi}{12}\cos \frac{\pi}{12} )}+ \frac{2}{\sqrt{2}}\\ =2\sqrt{(1 +\sin \frac{\pi}{6})}+ \frac{2}{\sqrt{2}}\\ =2\sqrt{(1 +\frac{1}{2})}+ \frac{2}{\sqrt{2}}\\ =2\sqrt{(\frac{3}{2})}+ \frac{2}{\sqrt{2}}\\ =2(\frac{\sqrt3}{\sqrt2})+ \frac{2}{\sqrt{2}}\\ =\sqrt6+\sqrt2\)

 

 

 

 

LaTex

\sin \frac{\pi}{12} + \sin \frac{3\pi}{12} + \sin \frac{5\pi}{12} + \sin \frac{7\pi}{12} + \sin \frac{9\pi}{12} + \sin \frac{11\pi}{12}\\

=\sin \frac{\pi}{12} + \sin \frac{3\pi}{12} + \sin \frac{5\pi}{12} + \sin \frac{5\pi}{12} + \sin \frac{3\pi}{12} + \sin \frac{\pi}{12}\\

=2(\sin \frac{\pi}{12} + \sin \frac{3\pi}{12} + \sin \frac{5\pi}{12} )\\

=2(\sin \frac{\pi}{12} + \sin \frac{\pi}{4} + \cos \frac{\pi}{12} )\\

=2(\sin \frac{\pi}{12}   + \cos \frac{\pi}{12} )+2\sin \frac{\pi}{4}\\

=2\sqrt{(\sin \frac{\pi}{12}   + \cos \frac{\pi}{12} )^2}+ \frac{2}{\sqrt{2}}\\

=2\sqrt{(\sin^2 \frac{\pi}{12}   + \cos^2 \frac{\pi}{12} +2\sin \frac{\pi}{12}\cos \frac{\pi}{12} )}+ \frac{2}{\sqrt{2}}\\

=2\sqrt{(1 +2\sin \frac{\pi}{12}\cos \frac{\pi}{12} )}+ \frac{2}{\sqrt{2}}\\

=2\sqrt{(1 +\sin \frac{\pi}{6})}+ \frac{2}{\sqrt{2}}\\

=2\sqrt{(1 +\frac{1}{2})}+ \frac{2}{\sqrt{2}}\\

=2\sqrt{(\frac{3}{2})}+ \frac{2}{\sqrt{2}}\\

=2(\frac{\sqrt3}{\sqrt2})+ \frac{2}{\sqrt{2}}\\

=\sqrt6+\sqrt2

19 may 2020