Difference between revisions of "2019 AMC 10A Problems/Problem 21"

The following problem is from both the 2019 AMC 10A #21 and 2019 AMC 12A #18, so both problems redirect to this page.

Problem

A sphere with center $O$ has radius $6$. A triangle with sides of length $15, 15,$ and $24$ is situated in space so that each of its sides is tangent to the sphere. What is the distance between $O$ and the plane determined by the triangle?

$\textbf{(A) }2\sqrt{3}\qquad \textbf{(B) }4\qquad \textbf{(C) }3\sqrt{2}\qquad \textbf{(D) }2\sqrt{5}\qquad \textbf{(E) }5\qquad$

Diagram

3D $[asy] import graph3; import palette; size(200); currentprojection=orthographic(0,4,2); triple f(pair z) {return expi(z.x,z.y);} surface s=surface(f,(0,0),(pi,2pi),70,Spline); draw((0,-5/6,sqrt(5)/3)--(2,2/3,sqrt(5)/3)--(-2,2/3,sqrt(5)/3)--cycle); draw(s,mean(palette(s.map(zpart),Grayscale())),nolight); draw((2,2/3,sqrt(5)/3)--(-2,2/3,sqrt(5)/3)); [/asy]$ Plane through triangle. $[asy] draw((0,0)--(12,9)--(24,0)--cycle); draw((12,9)--(12,0), dashed); draw((11.5,0)--(11.5,0.5)--(12,0.5)); draw(circle((12,4),4)); draw((12,4)--(48/5, 36/5)); dot((12,4)); label("15", (6,9/2),NW); label("15", (18,9/2),NE); label("24", (12,-1),S); label("r",(54/5, 28/5), SW); [/asy]$ -programjames1

Solution

The triangle is placed on the sphere so that three sides are tangent to the sphere. The cross-section of the sphere created by the plane of the triangle is also the incircle of the triangle. To find the inradius, use $\text{area} = \text{inradius} \cdot \text{semiperimeter}$. The area of the triangle can be found by drawing an altitude from the vertex between sides with length $15$ to the midpoint of the side with length $24$. Pythagorean triples $9$ - $12$ - $15$ show the base is $24$ and height is 9. $\frac {\text{base} \cdot \text{height}} {2}$ can be used to find the area of the triangle, $108$. The semiperimeter is $\frac {15 + 15 + 24} {2} = 27$ After plugging into the equation $108 = \text{inradius} \cdot 27$, we get $\text{inradius} = 4$. Let the distance between $O$ and the triangle be $x$. Choose a point on the incircle and denote it $A$. $\overline{OA}$ is $6$ because it is the radius of the sphere. Point $A$ to the center of the incircle is length $4$ because it is the inradius of the incircle. By using Pythagorean Theorem, you will get that $x$ is $\sqrt{6^2-4^2}=\sqrt{20}\implies\boxed{\textbf {(D)} 2 \sqrt {5}}$.

- ViolinGod(Argonauts16 latex)