求值cos2π/9cos4π/9cos8π/9

问题描述:

求值cos2π/9cos4π/9cos8π/9

cos2π/9cos4π/9cos8π/9
分子分母同时乘以sin2π/9,
就行了
(8sin2π/9cos2π/9cos4π/9cos8π/9)/8sin2π/9
分子变为sin16π /9
所以为-1/8