OR^2 = (cosθ+sinθ)^2 + (cosθ)^2
= 1 + 2cosθ*sinθ + (cosθ)^2
= 1 + sin(2θ) + (1/2)*(cos(2θ) + 1)
= 3/2 + sin(2θ) + (1/2)*cos(2θ)
= 3/2 + √(5/4) * sin(2θ + α)

sinα= √(1/5) , cosα= √(4/5)

0＜2θ＜π , 0＜α＜π/2だから
-√(1/5) = -sinα＜sin(2θ + α)≦1

3/2 + √(5/4)*{-√(1/5)} ＜ OR^2 ≦ 3/2 + √(5/4)
1 ＜ OR^2 ≦ 3/2 + (√5)/2 = (6+2√5)/4 = (√5 + 1)^2/4
1 ＜ OR ≦ (√5 + 1)/2