Graph of the integrand β(θ)F(β(θ)) (thick black curve) defined in Eq. (28) for the “unfavorable” case |θ_B | → θ_r. This integrand is composed of the functions β(θ) (solid gray curve) and F(β(θ)) (dashed graycurve). The (almost) symmetrical graph of β(θ) with respect to (θ_t, 0) and F(β(θ)) with respect to the axis θ = θ_t (dotted vertical line) implies the (almost) symmetry of the integrand with respect to (θ_t, 0). As a consequence, the two gray areas (almost) compensate each other implying that |ε_AB | ≪ θ_t, hence |η_AB|≪ 1, and confirms the validity of the Eq. (29).