20/10/01 07:16:33.66 n2o6aWK1.net
そりゃ、正5角形では辺・対角線の交角が36°の倍数だから…
θ = 72° とおくと
cos(48) + cos(36) - cos(12) - 1/2
= cos(120-θ) + cos(180-2θ) - cos(θ-60) - 1/2
= - cos(2θ) - cosθ - 1/2
= - {cos(-2θ) + cos(-θ) + cos(0) + cosθ + cos(2θ)}/2
= 0,
でもできるな
* {2(2xx-1) +2x +1}^2
= (4xx +2x -1)^2
= {T_5(x) -1} / (x-1),
x = cos(72) とおくと
(4xx +2x -1)^2 = {T_5(x) -1} / (x-1)
= {cos(360) -1} / {cos(72) -1}
= 0,