cos3π/11
=cos(π-8π/3)=cos(8π/3)
sin(16π/11）
= sin(π+5π/11)
=-sin(5π/11)
sin(10π/11）
= sin(π-π/11)
=sin(π/11)
(cosπ/11)(cos2π/11)(cos3π/11)(cos4π/11)(cos5π/11)
=-(cosπ/11)(cos2π/11)(cos4π/11)(cos8π/11)(cos5π/11)
=-(sinπ/11)(cosπ/11)(cos2π/11)(cos4π/11)(cos8π/11)(cos5π/11) /(sinπ/11)
=-(1/2)(sin2π/11)(cos2π/11)(cos4π/11)(cos8π/11)(cos5π/11) /(sinπ/11)
=-(1/4)(sin4π/11)(cos4π/11)(cos8π/11)(cos5π/11) /(sinπ/11)
=-(1/8)(sin8π/11)(cos8π/11)(cos5π/11) /(sinπ/11)
=-(1/16)(sin16π/11)(cos5π/11) /(sinπ/11)
=(1/16)(sin5π/11)(cos5π/11) /(sinπ/11)
=(1/32)(sin10π/11)/(sinπ/11)
=(1/32)(sinπ/11)/(sinπ/11)
=1/32