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2004 AMC 12B Problems/Problem 16

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Problem

A function f is defined by f(z) = i\overline{z}, where i=\sqrt{-1} and \overline{z} is the complex conjugate of z. How many values of z satisfy both |z| = 5 and f(z) = z?

\mathrm{(A)}\ 0\qquad\mathrm{(B)}\ 1\qquad\mathrm{(C)}\ 2 \qquad\mathrm{(D)}\ 4\qquad\mathrm{(E)}\ 8

Solution

Let z = a+bi, so \overline{z} = a-bi. By definition, z = a+bi = f(z) = i(a-bi) = b+ai, which implies that all solutions to f(z) = z lie on the line y=x on the complex plane. The graph of |z| = 5 is a circle centered at the origin, and there are 2 \Rightarrow \mathrm{(C)} intersections.

See also

2004 AMC 12B (ProblemsResources)
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
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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