计算sin(π/10)x sin (13π/10)

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计算sin(π/10)xsin(13π/10)计算sin(π/10)xsin(13π/10)计算sin(π/10)xsin(13π/10)sin(π/10)xsin(13π/10)=-1/2[cos(

计算sin(π/10)x sin (13π/10)
计算sin(π/10)x sin (13π/10)

计算sin(π/10)x sin (13π/10)
sin(π/10)x sin (13π/10)
=-1/2[cos(π/10+13π/10)-cos(π/10-13π/10)]
=-1/2[cos(7π/5)-cos(-6π/5)]
=-1/2[cos(π+2π/5)-cos(-π-π/5)]
=-1/2[-cos2π/5+cosπ/5]