Find the value of cos42 + cos78 + cos162

Asked by Ayesha Maria Zahir | 21st Jul, 2013, 11:58: AM

Expert Answer:

cos42o + cos78o + cos162o

= cos42o + cos78o + cos(180o - 18o)

= cos42o + cos78o - cos18o

= 2cos[(42o + 78o)/2] cos[(78o - 42o)/2] - cos18o

= 2cos60o cos18o - cos18o

= cos18o - cos18o = 0

Answered by  | 21st Jul, 2013, 06:19: PM

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