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2008 AMC 12A Problems/Problem 23

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Problem

The solutions of the equation z^4+4z^3i-6z^2-4zi-i=0 are the vertices of a convex polygon in the complex plane. What is the area of the polygon?

\mathrm{(A)}\ 2^{\frac{5}{8}}\qquad\mathrm{(B)}\ 2^{\frac{3}{4}}\qquad\mathrm{(C)}\ 2\qquad\mathrm{(D)}\ 2^{\frac{5}{4}}\qqua...

Solution

Looking at the coefficients, we are immediately reminded of the binomial expansion of {\left(x+1\right)}^{4}.

Modifying this slightly, we can write the given equation as: {\left(x+i\right)}^{4}=1+i=2^{\frac{1}{2}}\cdot \text{cis}\, \frac {\pi}{4} We can apply a translation of -i and a rotation of -\frac{\pi}{4} (both operations preserve area) to simplify the problem: z^{4}=2^{\frac{1}{2}}

Because the roots of this equation are created by rotating \frac{\pi}{2} radians successively about the origin, the quadrilateral is a square.

We know that half the diagonal length of the square is {\left(2^{\frac{1}{2}}\right)}^{\frac{1}{4}}=2^{\frac{1}{8}}

Therefore, the area of the square is \frac{\left( 2 \cdot 2^{\frac{1}{8} \right)}^{2}}{2}=\frac{2^{\frac{9}{4}}}{2}=2^{\frac{5}{4}} \Rightarrow D.

See Also

2008 AMC 12A (ProblemsResources)
Preceded by
Problem 22
Followed by
Problem 24
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Looking for a challenging geometry text? Preparing for MATHCOUNTS or the AMC exams? Check out Art of Problem Solving's Introduction to Geometry by Richard Rusczyk.
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