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2002 AMC 12B Problem 24

Problem 24 of 25HarderAlgebraGeometry

A convex quadrilateral ABCDABCD with area 20022002 contains a point PP in its interior such that PA=24,PA=24, PB=32,PB=32, PC=28,PC=28, and PD=45.PD=45. Find the perimeter of ABCD.ABCD.

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Solution

For any quadrilateral, the area is at most 12d1d2\tfrac12\,d_1 d_2 where d1,d2d_1,d_2 are the diagonals, with equality exactly when they are perpendicular. Here 2002=Area12ACBD12(PA+PC)(PB+PD)=125277=2002. \begin{gathered} 2002=\text{Area} \\ {}\le \tfrac12\,AC\cdot BD \\ {}\le \tfrac12(PA+PC)(PB+PD) \\ {}= \tfrac12\cdot52\cdot77 \\ {}= 2002. \end{gathered} Equality forces the diagonals to be perpendicular and to intersect at P.P. Then AB=242+322=40,BC=282+322=4113, \begin{aligned} AB &= \sqrt{24^2+32^2}=40, \\ BC &= \sqrt{28^2+32^2}=4\sqrt{113}, \end{aligned} CD=282+452=53,DA=452+242=51. \begin{aligned} CD &= \sqrt{28^2+45^2}=53, \\ DA &= \sqrt{45^2+24^2}=51. \end{aligned} The perimeter is 144+4113=4(36+113).144+4\sqrt{113}=4\left(36+\sqrt{113}\right). Thus, the correct answer is E.

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Concepts: area · diagonal · Pythagorean Theorem · bounding to limit cases

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