sin18分之派乘sin18分之5派乘sin负18分之65派

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sin18分之派乘sin18分之5派乘sin负18分之65派sin18分之派乘sin18分之5派乘sin负18分之65派sin18分之派乘sin18分之5派乘sin负18分之65派=sin(π/18)

sin18分之派乘sin18分之5派乘sin负18分之65派
sin18分之派乘sin18分之5派乘sin负18分
之65派

sin18分之派乘sin18分之5派乘sin负18分之65派
=sin(π/18)sin(5π/18)sin(-65π/18)
=sin(π/18)sin(5π/18)sin(11π/18)
=sin(π/18)sin(5π/18)sin(7π/18)
=sin(π/18)cos(2π/9)cos(π/9)
=[8cos(π/18)×sin(π/18)×cos(π/9)×cos(2π/9)]/[8cos(π/18)]
=[4sin(π/9)×cos(π/9)×cos(2π/9)]/\[8cos(π/18)]
=[sin(4π/9)]/[8cos(π/18)]
=[sin(4π/9)]/[8sin(4π/9)]
=1/8