Difference between revisions of "2015 AMC 10A Problems/Problem 24"

Latest revision as of 16:35, 26 May 2024

The following problem is from both the 2015 AMC 12A #19 and 2015 AMC 10A #24, so both problems redirect to this page.

Problem 24

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

$\textbf{(A) }30\qquad\textbf{(B) }31\qquad\textbf{(C) }61\qquad\textbf{(D) }62\qquad\textbf{(E) }63$

Solution 1

Let $BC = x$ and $CD = AD = y$ be positive integers. Drop a perpendicular from $A$ to $CD$ to show that, using the Pythagorean Theorem, that $x^2 + (y - 2)^2 = y^2.$ Simplifying yields $x^2 - 4y + 4 = 0$ , so $x^2 = 4(y - 1)$ . Thus, $y$ is one more than a perfect square.

The perimeter $p = 2 + x + 2y = 2y + 2\sqrt{y - 1} + 2$ must be less than 2015. Simple calculations demonstrate that $y = 31^2 + 1 = 962$ is valid, but $y = 32^2 + 1 = 1025$ is not. On the lower side, $y = 1$ does not work (because $x > 0$ ), but $y = 1^2 + 1$ does work. Hence, there are 31 valid $y$ (all $y$ such that $y = n^2 + 1$ for $1 \le n \le 31$ ), and so our answer is $\boxed{\textbf{(B) } 31}$

Solution 2

Let $BC = x$ and $CD = AD = z$ be positive integers. Drop a perpendicular from $A$ to $CD$ . Denote the intersection point of the perpendicular and $CD$ as $E$ .

$AE$ 's length is $x$ , as well. Call $ED$ $y$ . By the Pythagorean Theorem, $x^2 + y^2 = (y + 2)^2$ . And so: $x^2 = 4y + 4$ , or $y = (x^2-4)/4$ .

Writing this down and testing, it appears that this holds for all $x$ . However, since there is a dividend of 4, the numerator must be divisible by 4 to conform to the criteria that the side lengths are positive integers. In effect, $x$ must be a multiple of 2 to let the side lengths be integers. We test, and soon reach 62. It gives us $p = 1988$ , which is less than 2015. However, 64 gives us $2116 > 2015$ , so we know 62 is the largest we can go up to. Count all the even numbers from 2 to 62, and we get $\boxed{\textbf{(B) } 31}$ .

-jackshi2006

Video Solution by Richard Rusczyk

https://artofproblemsolving.com/videos/amc/2015amc10a/398

~ dolphin7

Video Solution

https://youtu.be/9DSv4zn7MyE

~savannahsolver

2015 AMC 10A (Problems • Answer Key • Resources)
Preceded by Problem 23	Followed by Problem 25
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 10 Problems and Solutions

2015 AMC 12A (Problems • Answer Key • Resources)
Preceded by Problem 18	Followed by Problem 20
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 12 Problems and Solutions

The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.

@@ Line 26: / Line 26: @@
 -jackshi2006
-==Solution 3==
-Let <math>AD=CD=a</math>. Construct point <math>E</math> on line <math>CD</math> so that <math>AE</math> is perpendicular to <math>CD</math>, <math>AE=BC=b</math>. <math>CE=AB=2</math>, <math>DE=a-2</math>.
-Because <math>\angle AED={90}^\circ</math>:
- <math>(a-2)^2+b^2=a^2</math>
- <math>a^2-4a+4+b^2=a^2</math>
- <math>b^2+4=4a</math>
- Note that from here we know that <math>b</math> must be even.
- <math>a=\frac{b^2+4}{4}</math>
-We also know that <math>p < 2015</math>:
- <math>p=AB+BC+CD+AD</math>
- <math>p=2+b+a+a</math>
- <math>p=2a+b+2</math>
- <math>2a+b+2<2015</math>
- <math>2a+b<2013</math>
-Substituting <math>a</math> in we get:
- <math>\frac{b^2+4}{2}+b<2013</math>
- <math>b^2+4+2b<4026</math>
- <math>b^2+2b+1<4023</math>
- <math>(b+1)^2<4023</math>
- <math>b+1<\sqrt{4023}</math>, b is an integer
- <math>b+1 \le 63</math>
- <math>b \le 62</math>
-From the triangle inequality:
- <math>a-2+b>a</math>
- <math>b>2</math>
-But, <math>\triangle ADE</math> does not have to exist. Quadrilateral <math>ABCD</math> could be a square, in that case <math>b=2</math>.
-So, <math>2 \le b \le 62</math> and <math>b</math> must be even. Count all the even numbers from <math>2</math> to <math>62</math>, <math>\frac{62-2}{2}+1=\boxed{\textbf{(B) } 31}</math>.
-~isabelchen
 == Video Solution by Richard Rusczyk ==

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