Cos(20°)cos(40°)cos(80°) = ?

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Answer 1 · 2017-10-05T08:56:09

# 1/8.#

#cos20^@cos40^@cos80^@,#

#=1/(2sin20^@)(2sin20^@cos20^@)cos40^@cos80^@,#

#=1/(2sin20^@)sin(2xx20^@)cos40^@cos80^@,#

#=1/(4sin20^@)(2sin40^@cos40^@)cos80^@,#

#=1/(4sin20^@)sin80^@cos80^@,#

#=1/(8sin20^@)(2sin80^@cos80^@),#

#=1/(8sin20^@)sin160^@,#

#=1/(8sin20^@)sin(180^@-20^@),#

#=1/(8sin20^@)sin20^@,#

#=1/8.#

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