2024 AMC 12B Problems/Problem 20

Problem 20

Suppose $A$ , $B$ , and $C$ are points in the plane with $AB=40$ and $AC=42$ , and let $x$ be the length of the line segment from $A$ to the midpoint of $\overline{BC}$ . Define a function $f$ by letting $f(x)$ be the area of $\triangle ABC$ . Then the domain of $f$ is an open interval $(p,q)$ , and the maximum value $r$ of $f(x)$ occurs at $x=s$ . What is $p+q+r+s$ ?

$\textbf{(A) }909\qquad \textbf{(B) }910\qquad \textbf{(C) }911\qquad \textbf{(D) }912\qquad \textbf{(E) }913\qquad$

Solution 1

Let the midpoint of $BC$ be $M$ , and let the length $BM = CM = a$ . We know there are limits to the value of $x$ , and these limits can probably be found through the triangle inequality. But the triangle inequality relates the third side length $BC$ to $AC$ and $AB$ , and doesn't contain any information about the median. Therefore we're going to have to write the side $BC$ in terms of $x$ and then use the triangle inequality to find bounds on $x$ .

We use Stewart's theorem to relate $BC$ to the median $AM$ : $man + dad = bmb + cnc$ . In this case $m = a$ , $n=a$ , $a = m+n$ , $d = x$ , $b = 42$ , $c = 40$ .

Therefore we get the equation $2a^3 + 2ax^2 = a \cdot 42^2 + a \cdot 40^2$

$2a^2 + 2x^2 = 42^2 + 40^2$ .

Notice that since $20-21-29$ is a pythagorean triple, this means $2a^2 + 2x^2 = 58^2$ .

$\implies a^2 = \frac{58^2}{2}-x^2$ $\implies a = \sqrt{\frac{58^2}{2}-x^2}$

By triangle inequality, $2a+40>42 \implies a>1$ and $40+42>2a \implies a<41$

Let's tackle the first inequality: $\sqrt{\frac{58^2}{2}-x^2}>1 \implies x^2 < \frac{58^2}{2}-1$

$\implies x^2 < \frac{40^2+42^2}{2}-1 \implies x^2<41^2$

Here we use the property that $\frac{x^2+(x+2)^2}{2}-1 = (x+1)^2$ .

Therefore in this case, $x<41$ .

For the second inequality, $\sqrt{\frac{58^2}{2}-x^2} < 41 \implies x^2 > \frac{58^2}{2}-41^2$

$\implies x^2 > \frac{58^2}{2}-1 + 1 - 41^2$

$\implies x^2 > 41^2 + 1 - 41^2 \implies x^2 > 1 \implies x > 1$

Therefore we have $1<x<41$ , so the domain of $f(x)$ is $(1,41)$ .

The area of this triangle is $\frac{1}{2} 42 \cdot 40 \cdot \sin(\theta)$ . The maximum value of the area occurs when the triangle is right, i.e. $\theta = 90^{\circ}$ . Then the area is $\frac{1}{2} \cdot 40 \cdot 42 = 840$ . The length of the median of a right triangle is half the length of it's hypotenuse, which squared is $40^2+42^2 = 58^2$ . Thus the length of $x$ is $29$ .

Our final answer is $1+41+840+29 = \boxed{\textbf{911 } (C)}$

~KingRavi

Solution 2

Let midpoint of $BC$ as $M$ , extends $AM$ to $D$ and $MD=x$ ,

triangle $ACD$ has $3$ sides $(40,42,2x)$ as such, $2< 2x < 82$ $1\le x \le41$ so $p = 1, q=41$

$2\cdot f(x) = 40 \cdot 42 \cdot \sin(A) \le 2\cdot840$ so $r = 840$ which is achieved when $A = 90^\circ$ , then $\angle ACD = 90^\circ$ $(2x)^2 = 40^2 + 42^2$ $x = 29$ $s= 29$ $p+q+s+r = 1 + 41 + 29 + 840 = \fbox{\textbf{(C) } 911}$

~luckuso

2024 AMC 12B (Problems • Answer Key • Resources)
Preceded by Problem 19	Followed by Problem 21
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2024 AMC 12B Problems/Problem 20

Contents

Problem 20

Solution 1

Solution 2

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