If 310 = r, then the value of cos O cos 20 cos 40 cos 80 cos 160 is
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cos 40 + cos 80 + cos 160 + cos 240
= (40+80) cos( 2cos( 2 2 80)+cos(180-20) + cos(180 +
60)
40-80
= 2cos60.cos(-20) + (-cos20) + (-cos60)
cos C+cosD=2cos( C+D 2 ).cos( C-D 2 )
cos(180 + theta) = - cos theta cos(180 - theta) = - cos theta
= 2 x / cos (20) - cos(20) - -/ (cos(-0) = cose)
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COSAD+ (0550 + (at 160+ 6 zice
2105 60. (TO) + (-10520) + (-60)
cos20 cos20 - -1/2 = -½
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