Small-Angle Approximation | Brilliant Math & Science Wiki A right triangle with two sides formed from the radii of a circle and the third side tangent to the circle. s^ {\prime} s′, the third side of the triangle. The small-angle approximation thus corresponds to. s\approx s^ {\prime} s ≈ s′ in this diagram. s ′ sin ⁡ θ = r sin ⁡ 9 0 ∘ = r s ′ = r sin ⁡ θ. = r s′ = rsinθ.