2002 AMC 12B Problem 24

Attempt Problem 24 of the 2002 AMC 12B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2002 AMC 12B solutions, or check the answer key.

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24.

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.

420024\sqrt{2002}

284652\sqrt{8465}

2(48+2002)2\left(48+\sqrt{2002}\right)

286332\sqrt{8633}

4(36+113)4\left(36+\sqrt{113}\right)

Answer: E
Concepts:areadiagonalPythagorean Theorembounding to limit cases
Difficulty rating: 2150
Small Hint:

For a quadrilateral with diagonals d1,d2,d_1,d_2, the area is at most 12d1d2,\tfrac12 d_1 d_2, with equality when the diagonals are perpendicular

Big Hint:

Here 12(PA+PC)(PB+PD)\tfrac12(PA+PC)(PB+PD) =2002=2002 forces perpendicular diagonals meeting at PP; find each side with the Pythagorean theorem

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.

Problem 23#23
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