[tex]( \cos( \alpha ) + \cos(2 \alpha ) + \cos(3 \alpha ) ) \div ( \sin( \alpha ) + \sin(2 \alpha ) + \sin(3 \alpha ) ) [/tex]
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сosa+cosb=2cos(a+b)/2*cos(a-b)/2
sina+sinb=2sin(a+b)/2*cos(a-b)/2
(2cos2acosa+cos2a)/(2sin2a*cosa+sin2a)=
=(cos2a/sin2a)((2cosa+1)/(2cosa+1))=cos2a/sin2a=ctg2a