In mathematics, Jordan's inequality, named after Camille Jordan, states that
2 π x ≤ sin ( x ) ≤ x for x ∈ [ 0 , π 2 ] . {\displaystyle {\frac {2}{\pi }}x\leq \sin(x)\leq x{\text{ for }}x\in \left[0,{\frac {\pi }{2}}\right].}
It can be proven through the geometry of circles (see drawing).
Notes
Further reading Serge Colombo: Holomorphic Functions of One Variable. Taylor & Francis 1983, ISBN 0677059507, p. 167-168 (online copy) Da-Wei Niu, Jian Cao, Feng Qi: Generealizations of Jordan's Inequality and Concerned Relations. U.P.B. Sci. Bull., Series A, Volume 72, Issue 3, 2010, ISSN 1223-7027 Feng Qi: Jordan's Inequality: Refinements, Generealizations, Applications and related Problems Archived 2016-03-03 at the Wayback Machine. RGMIA Res Rep Coll (2006), Volume: 9, Issue: 3, Pages: 243–259 Meng-Kuang Kuo: Refinements of Jordan's inequality. Journal of Inequalities and Applications 2011, 2011:130, doi:10.1186/1029-242X-2011-130
External links Jordan's inequality at the Proof Wiki Jordan's and Kober's inequalities at cut-the-knot.org Weisstein, Eric W. "Jordan's inequality". MathWorld.