Difference between revisions of "2011 AMC 10B Problems/Problem 20"
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== Solution #2== | == Solution #2== | ||
− | We follow the steps shown above until we draw pentagon <math>BIHFE</math>. We know that rhombus <math>ABCD</math> can be divided into equilateral triangles <math>\triangle ABD</math> and <math>\triangle CBD</math>. Using the 30-60-90 triangle rules, we find the height of the equilateral triangles (and the height of the rhombus) to be <math>\sqrt{3}</math>. Therefore, the area of <math>ABCD</math> is <math>2\sqrt{3}</math>. We now have to take off the areas <math>\triangle CDA</math>, <math>\triangle CEF</math>, and <math>\triangle AIH</math> to get the desired shape. <math>\triangle CDA</math> is just half of <math>ABCD</math> | + | We follow the steps shown above until we draw pentagon <math>BIHFE</math>. We know that rhombus <math>ABCD</math> can be divided into equilateral triangles <math>\triangle ABD</math> and <math>\triangle CBD</math>. Using the 30-60-90 triangle rules, we find the height of the equilateral triangles (and the height of the rhombus) to be <math>\sqrt{3}</math>. Therefore, the area of <math>ABCD</math> is <math>2\sqrt{3}</math>. We now have to take off the areas <math>\triangle CDA</math>, <math>\triangle CEF</math>, and <math>\triangle AIH</math> to get the desired shape. <math>\triangle CDA</math> is just half of <math>ABCD</math> <math>(\sqrt {3})</math> and <math>\triangle AIH</math> and <math>\triangle CEF</math> are each <math>\frac{\sqrt {3}}{6}</math>, for a total area of <math>2\sqrt {3}-\sqrt {3}-\frac{\sqrt{3}}{6}-\frac{\sqrt{3}}{6}=\boxed{(C)\frac{2\sqrt{3}}{3}}</math>. |
== See Also== | == See Also== |
Revision as of 20:46, 16 March 2018
- The following problem is from both the 2011 AMC 12B #16 and 2011 AMC 10B #20, so both problems redirect to this page.
Contents
Problem
Rhombus has side length and °. Region consists of all points inside the rhombus that are closer to vertex than any of the other three vertices. What is the area of ?
Solution
Suppose that is a point in the rhombus and let be the perpendicular bisector of . Then if and only if is on the same side of as . The line divides the plane into two half-planes; let be the half-plane containing . Let us define similarly and . Then is equal to . The region turns out to be an irregular pentagon. We can make it easier to find the area of this region by dividing it into four triangles:
Since and are equilateral, contains , contains and , and contains . Then with and so . Multiply this by 4 and it turns out that the pentagon has area .
Solution #2
We follow the steps shown above until we draw pentagon . We know that rhombus can be divided into equilateral triangles and . Using the 30-60-90 triangle rules, we find the height of the equilateral triangles (and the height of the rhombus) to be . Therefore, the area of is . We now have to take off the areas , , and to get the desired shape. is just half of and and are each , for a total area of .
See Also
2011 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 19 |
Followed by Problem 21 | |
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 |
2011 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 15 |
Followed by Problem 17 |
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 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.