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Grade 12th passTrigonometry

cos 35 cos 85 cos 155=?

Profile image of sumanth
11 Years agoGrade 12th pass
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1 Answer

Profile image of Lab Bhattacharjee
11 Years ago
cos(155)=cos(180-25)=-cos(25)
 
So, cos35 cos85 cos155 =-cos25 cos(60-25)cos(60+25)
 
Now prove 4cosx cos(60-x) cos(60+x)=cos3x
 
cos75=cos(45+30)=...
 
 

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