计算：cosπ/15×cos2π/15×cos4π/15×cos8π/15 要过程
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cosπ/15×cos2π/15×cos4π/15×cos8π/15
=（sinπ/15×cosπ/15×cos2π/15×cos4π/15×cos8π/15）/（sinπ/15）
=（1/2sin2π/15×cos2π/15×cos4π/15×cos8π/15）/（sinπ/15）
=（1/4sin4π/15×cos4π/15×cos8π/15）/（sinπ/15）
=（1/8sin8π/15×cos8π/15）/（sinπ/15）
=（1/16sin16π/15）/sin(16π/15-π）
=（1/16sin16π/15）/（-sin16π/15）
=-1/16
cosπ/15×cos2π/15×cos4π/15×cos8π/15
=（sinπ/15×cosπ/15×cos2π/15×cos4π/15×cos8π/15）/（sinπ/15）
=（1/2sin2π/15×cos2π/15×cos4π/15×cos8π/15）/（sinπ/15）
=（1/4sin4π/15×cos4π/15×cos8π/15）/（sinπ/15）
=（1/8sin8π/15×cos8π/15）/（sinπ/15）
=（1/16sin16π/15）/sin(16π/15-π）
=（1/16sin16π/15）/（-sin16π/15）
=-1/16
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乘sin§/15
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