Difference between revisions of "2023 AMC 8 Problems/Problem 24"

Revision as of 03:17, 25 January 2023

Problem

Isosceles $\triangle ABC$ has equal side lengths $AB$ and $BC$ . In the figure below, segments are drawn parallel to $\overline{AC}$ so that the shaded portions of $\triangle ABC$ have the same area. The heights of the two unshaded portions are 11 and 5 units, respectively. What is the height of $h$ of $\triangle ABC$ ?

Add asymptote diagram*

(note: diagrams are not necessarily drawn to scale)

$\textbf{(A) } 14.6 \qquad \textbf{(B) } 14.8 \qquad \textbf{(C) } 15 \qquad \textbf{(D) } 15.2 \qquad \textbf{(E) } 15.4$

Solution 1

First, we notice that the smaller isosceles triangles are similar to the larger isosceles triangles. We can find that the area of the gray area in the first triangle is $[\text{ABC}]\cdot\left(1-\left(\tfrac{11}{h}\right)^2\right)$ . Similarly, we can find that the area of the gray part in the second triangle is $[\text{ABC}]\cdot\left(\tfrac{h-5}{h}\right)^2$ . These areas are equal, so $1-\left(\frac{11}{h}\right)^2=\left(\frac{h-5}{h}\right)^2$ . Simplifying yields $10h=146$ so $h=\boxed{\textbf{(A) }14.6}$ .

~MathFun1000 (~edits apex304)

Video Solution 1 by OmegaLearn (Using Similarity)

https://youtu.be/almtw4n-92A

Video Solution 2 by SpreadTheMathLove(Using Area-Similarity Relaitionship)

https://www.youtube.com/watch?v=GTlkTwxSxgo

2023 AMC 8 (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 AJHSME/AMC 8 Problems and Solutions

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

@@ Line 2: / Line 2: @@
 Isosceles <math>\triangle ABC</math> has equal side lengths <math>AB</math> and <math>BC</math>. In the figure below, segments are drawn parallel to <math>\overline{AC}</math> so that the shaded portions of <math>\triangle ABC</math> have the same area. The heights of the two unshaded portions are 11 and 5 units, respectively. What is the height of <math>h</math> of <math>\triangle ABC</math>?
-<asy>
+*Add asymptote diagram*
-// Diagram by TheMathGuyd
-size(9cm);
-real h = 2.5; // height
-real g=4; //c2c space
-real s = 0.65; //Xcord of Hline
-pair A,B,C;
-B=(0,h);
-C=(1,0);
-A=-conj(C);
-pair P=(s,h*(1-s)); //Endpoint of Hline
-path L=P--(-conj(P)); //Hline
-path T=A--B--C--cycle; //Triangle
-draw(L);
-draw(T);
-label("$A$",A,SW);
-label("$B$",B,N);
-label("$C$",C,SE);
-draw(shift(g,0)*L);
-draw(shift(g,0)*T);
-label("$A$",shift(g,0)*A,SW);
-label("$B$",shift(g,0)*B,N);
-label("$C$",shift(g,0)*C,SE);
-draw(B--shift(g,0)*B,dashed);
-draw(C--shift(g,0)*A,dashed);
-draw((g/2,0)--(g/2,h),dashed);
-draw((0,h*(1-s))--B,dashed);
-draw((g,h*(1-s))--(g,0),dashed);
-label("$5$", midpoint((g,h*(1-s))--(g,0)),UnFill);
-label("$h$", midpoint((g/2,0)--(g/2,h)),UnFill);
-label("$11$", midpoint((0,h*(1-s))--B),UnFill);
-</asy>
 (note: diagrams are not necessarily drawn to scale)

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