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Show that the Trigonometrical equationn Cos 42 + cos 78 + cos 162 = 0 ?

Mounik , 9 Years ago
Grade 11
anser 2 Answers
Mounik

Last Activity: 9 Years ago

The answer is take Lhs=
LHS = Cos 42 + cos 78 + cos 162
= cos(60 – 18)+ cos(60+18)+cos(180-18)
= cos 60 . cos 18 + sin 60 . sin 18 + cos 60 . cos 18 – sin60.sin 18 – cos 18
= 2cos 60 . cos 18 – cos 18
= 2. ½ . cos 18 – cos 18 
= 0 =RHS 
siva prasad reddy

Last Activity: 9 Years ago

LHS = Cos 42 + cos 78 + cos 162= cos(60 – 18)+ cos(60+18)+cos(180-18)= cos 60 . cos 18 + sin 60 . sin 18 + cos 60 . cos 18 – sin60.sin 18 – cos 18= 2cos 60 . cos 18 – cos 18= 2. ½ . cos 18 – cos 18 = 0 =RHS
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