这个等于多少呢
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这个等于多少呢这个等于多少呢
这个等于多少呢
=(-1/2)【cos(π/3)-cos(4π/18)】sin(7π/18)
=(-1/2)【(1/2)-cos(4π/18)】sin(7π/18)
=(-1/4)sin(7π/18)-(-1/2)cos(4π/18)sin(7π/18)
=(-1/4)sin(7π/18)-(-1/4)【sin(11π/18)-sin(-3π/18)】
=(-1/4)sin(7π/18)-(-1/4)sin(11π/18)+(-1/4)sin(-π/6)
=(-1/4)sin(7π/18)-(-1/4)sin(7π/18)+(1/4)sin(π/6)
=0+(1/4)×(1/2)
= 1/8