179. Prove that cos12° + cos84° + cos132°+cos156=_1/2
Answers
Answered by
20
Step-by-step explanation:
=cos 12°+cos84°+cos132°+cos156°
=(cos12°+cos132°)+(cos84°+cos156°)
=2cos 72°cos60+2cos 120°cos36°
=cos72°-cos36°=sin18°-cos36°
=(root5-1/4)-(root5+1/4)
= -1/2
Answered by
0
Step-by-step explanation:
= ( cos 12 degree + cos 132 degree) + ( cos 84 degree + cos 150degree)
= cos 72 degree - cos 36 degree
= sin18 degree - cos 30 degree
= -1/2
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