Difference between revisions of "2018 AMC 12B Problems/Problem 16"
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The answer is the same if we consider <math>z^8=81.</math> Now we just need to find the area of the triangle bounded by <math>\sqrt 3i, \sqrt 3,</math> and <math>\frac{\sqrt 3}{\sqrt 2}+\frac{\sqrt 3}{\sqrt 2}i.</math> This is just <math>\boxed{\textbf{B.}}</math> | The answer is the same if we consider <math>z^8=81.</math> Now we just need to find the area of the triangle bounded by <math>\sqrt 3i, \sqrt 3,</math> and <math>\frac{\sqrt 3}{\sqrt 2}+\frac{\sqrt 3}{\sqrt 2}i.</math> This is just <math>\boxed{\textbf{B.}}</math> | ||
| + | == Solution 2 (Understanding the polygon) == | ||
| + | |||
| + | The polygon formed will be a regular octagon since there are <math>8</math> roots of <math>z^8=81</math>. By normal math computation, we can figure out that two roots of <math>z^8=81</math> are <math>\sqrt{3}</math> and <math>-\sqrt{3}</math>. These will lie on the real axis of the plane. Since it's a regular polygon, there has to be points on the vertical plane also which will be <math>\sqrt{3}i</math> and <math>-\sqrt{3}i</math>. | ||
| + | |||
| + | Clearly, the rest of the points will lie in each quadrant. The next thing is to get their coordinates (note that to answer this question, we do not need all the coordinates, only 3 consecutive ones are needed). | ||
| + | |||
| + | The circumcircle of the octagon will have the equation <math>i^2+r^2=3</math>. The coordinates of the point in the first quadrant will be equal in magnitude and both positive, so <math>i=r</math>. Solving gives <math>i=r=\frac{\sqrt{3}}{\sqrt{2}}</math> (meaning that the root represented is <math>\frac{\sqrt{3}}{\sqrt{2}}+\frac{\sqrt{3}}{\sqrt{2}}i</math>). | ||
| + | |||
| + | This way we can deduce the values of the <math>8</math> roots of the equation to be <math>\sqrt{3},-\sqrt{3},-\sqrt{3}i,\sqrt{3}i,\frac{\sqrt{3}}{\sqrt{2}}+\frac{\sqrt{3}}{\sqrt{2}}i,-\frac{\sqrt{3}}{\sqrt{2}}-\frac{\sqrt{3}}{\sqrt{2}}i,\frac{\sqrt{3}}{\sqrt{2}}-\frac{\sqrt{3}}{\sqrt{2}}i,-\frac{\sqrt{3}}{\sqrt{2}}+\frac{\sqrt{3}}{\sqrt{2}}i</math>. | ||
| + | |||
| + | To get the area, <math>3</math> consecutive points such as <math>\sqrt{3},</math> <math>\frac{\sqrt{3}}{\sqrt{2}}+\frac{\sqrt{3}}{\sqrt{2}}i,</math> and <math>\sqrt{3}i</math> can be used. The area can be computed using different methods like using the shoelace formula, or subtracting areas to find the area. | ||
| + | The answer you get is <math>\boxed{B}</math>. | ||
| + | |||
| + | (This method is not actually as long as it seems if you understand what you're doing while doing it. Also calculations can be made a little easier by solving using <math>x^8=1</math> and multiplying your answer by <math>\sqrt{3}</math>). | ||
| + | |||
| + | |||
| + | ~OlutosinNGA | ||
== See Also == | == See Also == | ||
Revision as of 21:05, 8 April 2019
Problem
The solutions to the equation 
are connected in the complex plane to form a convex regular polygon, three of whose vertices are labeled 
and 
. What is the least possible area of 
Solution
The answer is the same if we consider 
Now we just need to find the area of the triangle bounded by 
and 
This is just 
Solution 2 (Understanding the polygon)
The polygon formed will be a regular octagon since there are 
roots of 
. By normal math computation, we can figure out that two roots of are 
and 
. These will lie on the real axis of the plane. Since it's a regular polygon, there has to be points on the vertical plane also which will be 
and 
.
Clearly, the rest of the points will lie in each quadrant. The next thing is to get their coordinates (note that to answer this question, we do not need all the coordinates, only 3 consecutive ones are needed).
The circumcircle of the octagon will have the equation 
. The coordinates of the point in the first quadrant will be equal in magnitude and both positive, so 
. Solving gives 
(meaning that the root represented is 
).
This way we can deduce the values of the roots of the equation to be 
.
To get the area, 
consecutive points such as 
and can be used. The area can be computed using different methods like using the shoelace formula, or subtracting areas to find the area.
The answer you get is 
.
(This method is not actually as long as it seems if you understand what you're doing while doing it. Also calculations can be made a little easier by solving using 
and multiplying your answer by ).
~OlutosinNGA
See Also
| 2018 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. 
