Small-angle approximation

For small angles, the trigonometric functions sine, cosine, and tangent can be calculated with reasonable accuracy by the following simple approximations: sin ⁡ θ ≈ tan ⁡ θ ≈ θ , cos ⁡ θ ≈ 1 − 1 2 θ 2 ≈ 1 , {\displaystyle {\begin{aligned}\sin \theta &\approx \tan \theta \approx \theta ,\\[5mu]\cos \theta &\approx 1-{\tfrac {1}{2}}\theta ^{2}\approx 1,\end{aligned}}} provided the angle is measured in radians. Angles measured in degrees must first be converted to radians by multiplying them by ⁠ π / 180 {\displaystyle \pi /180} ⁠.

Source: Wikipedia — Small-angle approximation (CC BY-SA 4.0)