(2)三倍角因式积:
sin3θ=3sinθ-4sin3θ
=3sinθ[1-(4/3)sin2θ]
=3sinθ[1-sin2θ/sin2(π/3)]
[由3θ=±π,得θ=±π/3]
(3)五倍角因式积:
sin5θ=5sinθ-20sin3θ+16sin5θ
=5sinθ[1-4sin2θ+(16/5)sin4θ]
=5sinθ[1-sin2θ/sin2(π/5)][1-sin2θ/sin2(2π/5)]
[由5θ=±π、±2π,得θ=±π/5、±2π/5]
(4)奇倍角因式积:依上类推
sin(2n+1)θ=sinθ∑(k=0…n)(-1)kC(2n+1,2k+1)(1-sin2θ)n-ksin2kθ
=(2n+1)sinθ∏(k=1…n){1-sin2θ/sin2[kπ/(2n+1)]}
[由(2n+1)θ=±kπ,得θ=±kπ/(2n+1),k=1、2、……、n]
(5)换元:令(2n+1)θ=x,得
sinx=(2n+1)sin[x/(2n+1)]∏(k=1…n){1-sin2[x/(2n+1)]/sin2[kπ/(2n+1)]}
(6)取极限:令n→∞ 得