Problem A. 571. (October 2012)
A. 571. The side lengths of a triangle are a, b, and c. Prove that
Proposed by: Daniel Campos, San Jose, Costa Rica
Deadline expired on 12 November 2012.
Solution. Notice that we have equality in the degenerate case when one of a,b,c is zero and the other two are equal.
We show that
(This estimate preserves equality in the case mentioned.)
By the cyclic permutations of the variables we get and . The sum of these and (1) is the statement of the problem.