Difference between revisions of "2021 Fall AMC 12B Problems/Problem 9"

Revision as of 20:34, 9 January 2022

Problem

Triangle $ABC$ is equilateral with side length $6$ . Suppose that $O$ is the center of the inscribed circle of this triangle. What is the area of the circle passing through $A$ , $O$ , and $C$ ?

$\textbf{(A)} \: 9\pi \qquad\textbf{(B)} \: 12\pi \qquad\textbf{(C)} \: 18\pi \qquad\textbf{(D)} \: 24\pi \qquad\textbf{(E)} \: 27\pi$

Solution 1 (Cosine Rule)

Construct the circle that passes through $A$ , $O$ , and $C$ , centered at $X$ .

Also notice that $\overline{OA}$ and $\overline{OC}$ are the angle bisectors of angle $\angle BAC$ and $\angle BCA$ respectively. We then deduce $\angle AOC=120^\circ$ .

Consider another point $M$ on Circle $X$ opposite to point $O$ .

As $AOCM$ is an inscribed quadrilateral of Circle $X$ , $\angle AMC=180^\circ-120^\circ=60^\circ$ .

Afterward, deduce that $\angle AXC=2·\angle AMC=120^\circ$ .

By the Cosine Rule, we have the equation: (where $r$ is the radius of circle $X$ )

$2r^2(1-\cos(120^\circ))=6^2$

$r^2=12$

The area is therefore $\pi r^2 = \boxed{\textbf{(B)}\ 12\pi}$ .

~Wilhelm Z

Solution 2

We have $\angle AOC = 120^\circ$ .

Denote by $R$ the circumradius of $\triangle AOC$ . In $\triangle AOC$ , the law of sines implies $2 R = \frac{AC}{\sin \angle AOC} = 4 \sqrt{3} .$

Hence, the area of the circumcircle of $\triangle AOC$ is $\pi R^2 = 12 \pi .$

Therefore, the answer is $\boxed{\textbf{(B) }12 \pi}$ .

~Steven Chen (www.professorchenedu.com)

Solution 3

As before, construct the circle that passes through $A$ , $O$ , and $C$ , centered at $X$ . Let $Y$ be the intersection of $\overline{OX}$ and $\overline{AB}$ .

Note that since $\overline{OA}$ is the angle bisector of $\angle BAC$ that $\angle OAC=30^\circ$ . Also by symmetry, $\overline{OX}$ $\perp$ $\overline{AB}$ and $AY = 3$ . Thus $\tan(30^\circ) = \frac{OY}{3}$ so $OY = \sqrt{3}$ .

Let $r$ be the radius of circle $X$ , and note that $AX = OX = r$ . So $\triangle AYX$ is a right triangle with legs

-SharpeMind

@@ Line 49: / Line 49: @@
 ~Steven Chen (www.professorchenedu.com)
+==Solution 3 ==
+As before, construct the circle that passes through <math>A</math>, <math>O</math>, and <math>C</math>, centered at <math>X</math>. Let <math>Y</math> be the intersection of <math>\overline{OX}</math> and <math>\overline{AB}</math>.
+Note that since <math>\overline{OA}</math> is the angle bisector of <math>\angle BAC</math> that <math>\angle OAC=30^\circ</math>. Also by symmetry, <math>\overline{OX}</math> <math>\perp</math> <math>\overline{AB}</math> and <math>AY = 3</math>. Thus <math>\tan(30^\circ) = \frac{OY}{3}</math> so <math>OY = \sqrt{3}</math>.
+Let <math>r</math> be the radius of circle <math>X</math>, and note that <math>AX = OX = r</math>. So <math>\triangle AYX</math> is a right triangle with legs
+-SharpeMind
 ==See Also==
 {{AMC12 box|year=2021 Fall|ab=B|num-a=10|num-b=8}}
 {{MAA Notice}}

2021 Fall AMC 12B (Problems • Answer Key • Resources)
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