But the arithmetic mean-geometric mean inequality insures that the coefficients of p, q, and r are each at least 2, from which the desired result follows.
We conclude with several comments about the lemma and the Erdo ̋s-Mordell inequality and their relationships to other results.
1. The three inequalities in the lemma are equalities if and only if O is the
center of the circumscribed circle of ABC. This follows from the observation
that the trapezoid in Figure 2(b) is a rectangle if and only if β + α2 = π and π2
γ + α1 = 2 (and similarly in the other two cases), so that ∠AOQ = β = ∠COQ. Hence the right triangles AOQ and COQ are congruent, and x = z. Similarly one can show that x = y. Hence, x = y = z and O must be the circumcenter of ABC. The coefficients of p, q, and r in (1) are equal to 2 if and only if a = b = c. Consequently we have equality in the Erdo ̋s-Mordell inequality if and only if ABC is equilateral and O is its center.