三角函数题求解，请看图片。
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cosπ/9*cos2π/9*cos4π/9
=[sinπ/9*cosπ/9*cos2π/9*cos4π/9]/sinπ/9
=[1/2sin2π/9*cos2π/9*cos4π/9]/sinπ/9
=[1/4sin4π/9*cos4π/9]/sinπ/9
=[1/8sin8π/9]/sinπ/9
=[1/8sinπ/9]/[sinπ/9]
=1/8
log2cosπ/9+log2cos2π/9+log2cos4π/9
=log2(cosπ/9*cos2π/9*cos4π/9)
=log2(1/8)
=-3
=[sinπ/9*cosπ/9*cos2π/9*cos4π/9]/sinπ/9
=[1/2sin2π/9*cos2π/9*cos4π/9]/sinπ/9
=[1/4sin4π/9*cos4π/9]/sinπ/9
=[1/8sin8π/9]/sinπ/9
=[1/8sinπ/9]/[sinπ/9]
=1/8
log2cosπ/9+log2cos2π/9+log2cos4π/9
=log2(cosπ/9*cos2π/9*cos4π/9)
=log2(1/8)
=-3
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