In mathematics, Ono's inequality is a theorem about triangles in the Euclidean plane. In its original form, as conjectured by T. Ono in 1914, the inequality is actually false; however, the statement is true for acute triangles and right triangles, as shown by F. Balitrand in 1916.
Statement of the inequality
Consider an acute or right triangle in the Euclidean plane with side lengths a, b and c and area A. Then
This inequality fails for general triangles (to which Ono's original conjecture applied), as shown by the counterexample