2015 AMC 12A Problem 19

Attempt Problem 19 of the 2015 AMC 12A 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 2015 AMC 12A solutions, or check the answer key.

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19.

For some positive integers p,p, there is a quadrilateral ABCDABCD with positive integer side lengths, perimeter p,p, right angles at BB and C,C, AB=2,AB = 2, and CD=AD.CD = AD. How many different values of p<2015p \lt 2015 are possible?

3030

3131

6161

6262

6363

Answer: B
Concepts:Pythagorean Theoremcounting integers in a range
Difficulty rating: 2010
Solution:

In every such quadrilateral CDAB.CD \ge AB. Let EE be the foot of the perpendicular from AA to CD;\overline{CD}; then CE=2CE = 2 and AE=BC.AE = BC. Let x=AEx = AE and y=DE,y = DE, so AD=2+y.AD = 2 + y.

By the Pythagorean Theorem x2+y2=(2+y)2,x^2 + y^2 = (2+y)^2, so x2=4+4yx^2 = 4 + 4y and xx is even. Writing x=2zx = 2z gives y=z21,y = z^2 - 1, and the perimeter is x+2y+6=2z2+2z+4.x + 2y + 6 = 2z^2 + 2z + 4.

Increasing values z=1,2,3,z = 1, 2, 3, \dots give the required quadrilaterals with increasing perimeter. For z=31z = 31 the perimeter is 1988,1988, and for z=32z = 32 it is 2116.2116. Therefore there are 3131 possible values of p<2015.p \lt 2015.

Thus, the correct answer is B.

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