This is a Test of Mathematics Solution Subjective 116 (from ISI Entrance). The book, Test of Mathematics at 10+2 Level is Published by East West Press. This problem book is indispensable for the preparation of I.S.I. B.Stat and B.Math Entrance.
If A, B, C are the angles of a triangle, then show that
We apply Jensen’s Inequality and Arithmetic Mean – Geometric Mean inequality here. Since cosine function is concave in the interval , we have