向量2a + b = (2cosα + cosβ, 2sinα + sinβ)
向量a - 2b = (cosα - 2cosβ, sinα - 2sinβ)
|2a + b| = 4cos²α + 4cosαcosβ + cos²β + 4sin²α + 4sinαsinβ + sin²β = 5 + 4cosαcosβ + 4sinαsinβ
|b - 2a| = cos²α - 4cosαcosβ + 4cos²β + sin²α - 4sinαsinβ + 4sin²β = 5 - 4cosαcosβ - 4sinαsinβ
5 + 4cosαcosβ + 4sinαsinβ = 5 - 4cosαcosβ - 4sinαsinβ
cosαcosβ + sinαsinβ = cos(β - α) = 0
β - α = π/2