Difference between revisions of "2017 AMC 10A Problems/Problem 22"
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(same diagram as Solution 1) | (same diagram as Solution 1) | ||
− | WLOG, let the side length of the triangle be <math>1</math>. Then, the area of the triangle is <math>\frac{\sqrt{3}}{4}</math>. We are looking for the area of the portion inside the triangle but outside the circle divided by the area of the triangle. Since <math>m\angle ABO = 90^{\circ}=m\angle ACO = 90^{\circ}</math>, and <math>m\angle ABC = m\angle ACB = 60^{\circ}</math>, we know <math>m\angle OBC=m\angle OCB=30^{\circ}</math>, and <math>m\angle BOC = 120^{\circ}</math>. Drop an angle bisector of <math>O</math> onto <math>BC</math>, call the point of intersection <math>D</math>. By SAS congruence, <math>\triangle BDO \cong \triangle CDO</math>, by CPCTC (Congruent Parts of Congruent Triangles are Congruent) <math>BD \cong DC</math> and they both measure <math>frac{1}{2}</math>. By 30-60-90 triangle, <math>OC = BO = \frac{\sqrt{3}}{3}</math>. The area of the region bounded by arc BC is one-third the area of circle O, whose area is <math>\left(\frac{\sqrt{3}}{3}\right)^{2}\pi=\frac{1}{3}\pi</math>. Therefore, the area of the region bounded by arc BC is <math>\frac{1}{3}\cdot\frac{1}{3}\pi=\frac{\pi}{9}</math>. We are nearly there. By 30-60-90 triangle, we know <math>DO = \frac{\sqrt{3}}{6}</math>, so the area of <math>\triangle BOC</math> is <math>\frac{1\cdot\frac{\sqrt{3}}{6}}{2}=\frac{\sqrt{3}}{12}</math>. The area of the region inside both the triangle and circle is the area of the region bounded by arc BC minus the area of <math>\triangle BOC</math>: <math>\frac{\pi}{9}-\frac{\sqrt{3}}{12}</math>. The area of the region outside of the circle but inside the triangle is <math>\frac{\sqrt{3}}{4}-\left(\frac{\pi}{9}-\frac{\sqrt{3}}{12}\right)=\frac{\sqrt{3}}{4}-\frac{\pi}{9}+\frac{\sqrt{3}}{12}=\frac{\sqrt{3}}{3}-\frac{\pi}{9}</math> and the ratio is <math>\frac{\frac{\sqrt{3}}{3}-\frac{\pi}{9}}{\frac{\sqrt{3}}{4}}=\left(\frac{\sqrt{3}}{3}-\frac{\pi}{9}\right)\cdot\frac{4}{\sqrt{3}}=\frac{4\sqrt{3}}{3\sqrt{3}}-\frac{4\pi}{9\sqrt{3}}=\frac{4}{3}-\frac{4\sqrt{3}\pi}{27} \Longrightarrow \boxed{\textbf{(E) } \frac 43 - \frac{4\sqrt 3 \pi}{27}}</math>. | + | WLOG, let the side length of the triangle be <math>1</math>. Then, the area of the triangle is <math>\frac{\sqrt{3}}{4}</math>. We are looking for the area of the portion inside the triangle but outside the circle divided by the area of the triangle. Since <math>m\angle ABO = 90^{\circ}=m\angle ACO = 90^{\circ}</math>, and <math>m\angle ABC = m\angle ACB = 60^{\circ}</math>, we know <math>m\angle OBC=m\angle OCB=30^{\circ}</math>, and <math>m\angle BOC = 120^{\circ}</math>. Drop an angle bisector of <math>O</math> onto <math>BC</math>, call the point of intersection <math>D</math>. By SAS congruence, <math>\triangle BDO \cong \triangle CDO</math>, by CPCTC (Congruent Parts of Congruent Triangles are Congruent) <math>BD \cong DC</math> and they both measure <math>\frac{1}{2}</math>. By 30-60-90 triangle, <math>OC = BO = \frac{\sqrt{3}}{3}</math>. The area of the region bounded by arc BC is one-third the area of circle O, whose area is <math>\left(\frac{\sqrt{3}}{3}\right)^{2}\pi=\frac{1}{3}\pi</math>. Therefore, the area of the region bounded by arc BC is <math>\frac{1}{3}\cdot\frac{1}{3}\pi=\frac{\pi}{9}</math>. We are nearly there. By 30-60-90 triangle, we know <math>DO = \frac{\sqrt{3}}{6}</math>, so the area of <math>\triangle BOC</math> is <math>\frac{1\cdot\frac{\sqrt{3}}{6}}{2}=\frac{\sqrt{3}}{12}</math>. The area of the region inside both the triangle and circle is the area of the region bounded by arc BC minus the area of <math>\triangle BOC</math>: <math>\frac{\pi}{9}-\frac{\sqrt{3}}{12}</math>. The area of the region outside of the circle but inside the triangle is <math>\frac{\sqrt{3}}{4}-\left(\frac{\pi}{9}-\frac{\sqrt{3}}{12}\right)=\frac{\sqrt{3}}{4}-\frac{\pi}{9}+\frac{\sqrt{3}}{12}=\frac{\sqrt{3}}{3}-\frac{\pi}{9}</math> and the ratio is <math>\frac{\frac{\sqrt{3}}{3}-\frac{\pi}{9}}{\frac{\sqrt{3}}{4}}=\left(\frac{\sqrt{3}}{3}-\frac{\pi}{9}\right)\cdot\frac{4}{\sqrt{3}}=\frac{4\sqrt{3}}{3\sqrt{3}}-\frac{4\pi}{9\sqrt{3}}=\frac{4}{3}-\frac{4\sqrt{3}\pi}{27} \Longrightarrow \boxed{\textbf{(E) } \frac 43 - \frac{4\sqrt 3 \pi}{27}}</math>. |
~JH. L | ~JH. L |
Revision as of 22:17, 13 June 2022
Problem
Sides and of equilateral triangle are tangent to a circle at points and respectively. What fraction of the area of lies outside the circle?
Solution 1
Let the radius of the circle be , and let its center be . Since and are tangent to circle , then , so . Therefore, since and are equal to , then (pick your favorite method) . The area of the equilateral triangle is , and the area of the sector we are subtracting from it is . The area outside of the circle is . Therefore, the answer is
Solution 2
(same diagram as Solution 1)
WLOG, let the side length of the triangle be . Then, the area of the triangle is . We are looking for the area of the portion inside the triangle but outside the circle divided by the area of the triangle. Since , and , we know , and . Drop an angle bisector of onto , call the point of intersection . By SAS congruence, , by CPCTC (Congruent Parts of Congruent Triangles are Congruent) and they both measure . By 30-60-90 triangle, . The area of the region bounded by arc BC is one-third the area of circle O, whose area is . Therefore, the area of the region bounded by arc BC is . We are nearly there. By 30-60-90 triangle, we know , so the area of is . The area of the region inside both the triangle and circle is the area of the region bounded by arc BC minus the area of : . The area of the region outside of the circle but inside the triangle is and the ratio is .
~JH. L
Video Solution
https://www.youtube.com/watch?v=GnJDNtjd57k&feature=youtu.be
https://youtu.be/ADDAOhNAsjQ -Video Solution by Richard Rusczyk
See Also
2017 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 21 |
Followed by Problem 23 | |
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 |
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