متراجحة پيدو

في الهندسة الرياضية، تنص متراجحة پيدو Pedoe's inequality المسماة على اسم دانيال پيدو على ما يلي: إذا كانت a، b، وc هي أطولا أضلاع مثلث له مساحة f وA، B، وC هي أطوال أضلاع مثلث آخر له مساحة F هندها تتحقق المتراجحة التالية:

$A^{2}(b^{2}+c^{2}-a^{2})+B^{2}(a^{2}+c^{2}-b^{2})+C^{2}(a^{2}+b^{2}-c^{2})\geq 16Ff,\,$

حيث في هذه المتراجحة حالة التساوي تكون محققة فقط وفقط إذا كان المثلثان متشابهان.

The expression on the left is not only symmetric under any of the six permutations of the set { (A, a), (B, b), (C, c) } of pairs, but also—perhaps not so obviously—remains the same if a is interchanged with A and b with B and c with C. In other words, it is a symmetric function of the pair of triangles.

Pedoe's inequality is a generalization of Weitzenböck's inequality, which is the case in which one of the triangles is equilateral.

Pedoe discovered the inequality in 1941 and published it subsequently in several articles. Later he learned that the inequality was already known in the 19th century to Neuberg, who however did not prove that the equality implies the similarity of the two triangles.

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 الإثبات

By Heron's formula, the area of the two triangles can be expressed as:

$16f^{2}=(a+b+c)(a+b-c)(a-b+c)(b+c-a)=(a^{2}+b^{2}+c^{2})^{2}-2(a^{4}+b^{4}+c^{4})$

$16F^{2}=(A+B+C)(A+B-C)(A-B+C)(B+C-A)=(A^{2}+B^{2}+C^{2})^{2}-2(A^{4}+B^{4}+C^{4})$

and then, using Cauchy-Schwarz inequality we have,

$16Ff+2a^{2}A^{2}+2b^{2}B^{2}+2c^{2}C^{2}$

$\leq {\sqrt {(16f^{2}+2a^{4}+2b^{4}+2c^{4})}}{\sqrt {(16F^{2}+2A^{4}+2B^{4}+2C^{4})}}$

$=(a^{2}+b^{2}+c^{2})(A^{2}+B^{2}+C^{2})$

So,

$16Ff\leq A^{2}(a^{2}+b^{2}+c^{2})-2a^{2}A^{2}+B^{2}(a^{2}+b^{2}+c^{2})-2b^{2}B^{2}+C^{2}(a^{2}+b^{2}+c^{2})-2c^{2}C^{2}$

$=A^{2}(b^{2}+c^{2}-a^{2})+B^{2}(a^{2}+c^{2}-b^{2})+C^{2}(a^{2}+b^{2}-c^{2})$

and the proposition is proven.

Equality holds if and only if ${\tfrac {a}{A}}={\tfrac {b}{B}}={\tfrac {c}{C}}={\sqrt {\tfrac {f}{F}}}$, that is, the two triangles are similar.

 انظر أيضاً

• List of triangle inequalities

 مراجع

• "A Two-Triangle Inequality", Daniel Pedoe, The American Mathematical Monthly, volume 70, number 9, page 1012, November, 1963.
• "An Inequality for Two Triangles", D. Pedoe, Proceedings of the Cambridge Philosophical Society, volume 38, part 4, page 397, 1943.

 وصلات خارجية

• Pedoe's inequality