In geometry, Barrow's inequality is an inequality relating the distances between certain points of a triangle. It states the following: Let P be a point inside the triangle ABC; U, V, and W be the points where the angle bisectors of BPC, CPA, and APB intersect the sides BC, CA, AB, respectively. Then
with equality holding only in the case of an equilateral triangle.
Barrow's inequality strengthens the Erdős–Mordell inequality, which has identical form except with PU, PV, and PW replaced by the three distances of P from the triangle's sides. It is named after David Francis Barrow. Barrow's proof of this inequality was published in 1937, as his solution to a problem posed in the American Mathematical Monthly of proving the Erdős–Mordell inequality. A simpler proof was later given by Mordell.
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