2015 AMC 10A Problem 24

Attempt Problem 24 of the 2015 AMC 10A 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 10A solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

24.

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 < 2015 are possible?

3030

3131

6161

6262

6363

Answer: B
Concepts:Pythagorean TheoremDiophantine Equationperimeter
Difficulty rating: 2300
Solution:

Let BC=xBC=x and CD=AD=yCD=AD=y. Dropping the altitude from AA to CDCD gives a right triangle with legs xx and y2y-2, and hypotenuse yy. Therefore (y2)2+x2=y2,(y-2)^2+x^2=y^2, so x2=4(y1)x^2=4(y-1).

Since xx is an integer, write x=2kx=2k. Then y=k2+1y=k^2+1, and the perimeter is p=2+x+y+y=2k2+2k+4. \begin{aligned} &p=2+x+y+y \\ &=2k^2+2k+4. \end{aligned}

We need 2k2+2k+4<20152k^2+2k+4<2015, or k2+k<1005.5k^2+k<1005.5. This holds for k=1,2,,31k=1,2,\ldots,31, while k=32k=32 is too large. Thus there are 3131 possible perimeters.

Thus, B is the correct answer.

← Problem 23#23
Full Exam

Problem 24 in Other Years