z为复数.建立恒等式1+z+z^2+z^3+···+z^n=[1-z^(n+1)]/(1-z)(z不等于1)并导出:1+cosθ+cos2θ+cos3θ+···+cosnθ=1/2+sin(n+1/2)θ/2sin(θ/2)(0

问题描述:

z为复数.建立恒等式1+z+z^2+z^3+···+z^n=[1-z^(n+1)]/(1-z)(z不等于1)并导出:1+cosθ+cos2θ+cos3θ+···+cosnθ=1/2+sin(n+1/2)θ/2sin(θ/2)(0

令z=e^jθ,代入恒等式,取实部就好