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Difference between revisions of "2005 AMC 12A Problems/Problem 20"

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Note: the values of x that satisfy <math>f^{[n]}(x) = \frac {1}{2}</math> are <math>\frac{1}{2^{n+1}}</math>, <math>\frac{3}{2^{n+1}}</math>, <math>\frac{5}{2^{n+1}}</math>, <math>\cdots</math> ,<math>\frac{2^{n+1}-1}{2^{n+1}}</math>.
 
Note: the values of x that satisfy <math>f^{[n]}(x) = \frac {1}{2}</math> are <math>\frac{1}{2^{n+1}}</math>, <math>\frac{3}{2^{n+1}}</math>, <math>\frac{5}{2^{n+1}}</math>, <math>\cdots</math> ,<math>\frac{2^{n+1}-1}{2^{n+1}}</math>.
  
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== Solution 2 (non-rigorous)==
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== Solution 2 (Engineers Induction)==
 
We are given that <math>f^{[2005]}(x) = \frac {1}{2}</math>. Thus,  <math>f(f^{[2004]}(x))=\frac{1}{2}</math>. Let <math>f^{[2004]}(x)</math> be equal to <math>y</math>. Thus <math>f(y)=\frac{1}{2}</math> or <math>y=\frac{1}{4}</math> or <math>\frac{3}{4}</math>. Now we know <math>f^{[2004]}(x)</math> is equal to <math>\frac{1}{4}</math> or <math>\frac{3}{4}</math>. Now we know that <math>f(f^{[2003]}(x))=\frac{1}{4}</math> or <math>\frac{3}{4}</math>. Now we solve for <math>f^{[2003]}(x)</math> and let  <math>f^{[2003]}(x)=z</math>. Thus <math>f(z)</math> is equal to <math>\frac{1}{8}</math>,<math>\frac{7}{8}</math>,<math>\frac{5}{8}</math>,and <math>\frac{3}{8}</math>. As we see, <math>f^{[2005]}(x)</math> has 1 solution, <math>f^{[2004]}(x)</math> has 2 solutions, and <math>f^{[2003]}(x)</math> has 4 solutions. Thus for each iteration we double the number of possible solutions. There are <math>2005</math> iterations and thus the number of solutions is <math>2^{2005}</math> <math>\Rightarrow\boxed{E}</math>
 
We are given that <math>f^{[2005]}(x) = \frac {1}{2}</math>. Thus,  <math>f(f^{[2004]}(x))=\frac{1}{2}</math>. Let <math>f^{[2004]}(x)</math> be equal to <math>y</math>. Thus <math>f(y)=\frac{1}{2}</math> or <math>y=\frac{1}{4}</math> or <math>\frac{3}{4}</math>. Now we know <math>f^{[2004]}(x)</math> is equal to <math>\frac{1}{4}</math> or <math>\frac{3}{4}</math>. Now we know that <math>f(f^{[2003]}(x))=\frac{1}{4}</math> or <math>\frac{3}{4}</math>. Now we solve for <math>f^{[2003]}(x)</math> and let  <math>f^{[2003]}(x)=z</math>. Thus <math>f(z)</math> is equal to <math>\frac{1}{8}</math>,<math>\frac{7}{8}</math>,<math>\frac{5}{8}</math>,and <math>\frac{3}{8}</math>. As we see, <math>f^{[2005]}(x)</math> has 1 solution, <math>f^{[2004]}(x)</math> has 2 solutions, and <math>f^{[2003]}(x)</math> has 4 solutions. Thus for each iteration we double the number of possible solutions. There are <math>2005</math> iterations and thus the number of solutions is <math>2^{2005}</math> <math>\Rightarrow\boxed{E}</math>
  

Latest revision as of 11:20, 18 October 2025

Problem

For each $x$ in $[0,1]$, define

\[f(x) = \begin{cases} 2x, \qquad\qquad \mathrm{if} \quad 0 \leq x \leq \frac{1}{2}\\ 2-2x, \qquad \mathrm{if} \quad \frac{1}{2} < x \leq 1. \end{cases}\]

Let $f^{[2]}(x) = f(f(x))$, and $f^{[n + 1]}(x) = f^{[n]}(f(x))$ for each integer $n \geq 2$. For how many values of $x$ in $[0,1]$ is $f^{[2005]}(x) = 1/2$?

$(\mathrm {A}) \ 0 \qquad (\mathrm {B}) \ 2005 \qquad (\mathrm {C})\ 4010 \qquad (\mathrm {D}) \ 2005^2 \qquad (\mathrm {E})\ 2^{2005}$

Solution 1

For the two functions $f(x)=2x,0\le x\le \frac{1}{2}$ and $f(x)=2-2x,\frac{1}{2}\le x\le 1$,as long as $f(x)$ is between $0$ and $1$, $x$ will be in the right domain, so we don't need to worry about the domain of $x$.

Also, every time we change $f(x)$, the expression for the final answer in terms of $x$ will be in a different form (although they'll all satisfy the final equation), so we get a different starting value of $x$. Every time we have two choices for $f(x$) and altogether we have to choose $2005$ times. Thus, $2^{2005}\Rightarrow\boxed{E}$.

Note: the values of x that satisfy $f^{[n]}(x) = \frac {1}{2}$ are $\frac{1}{2^{n+1}}$, $\frac{3}{2^{n+1}}$, $\frac{5}{2^{n+1}}$, $\cdots$ ,$\frac{2^{n+1}-1}{2^{n+1}}$.

Solution 2 (Engineers Induction)

We are given that $f^{[2005]}(x) = \frac {1}{2}$. Thus, $f(f^{[2004]}(x))=\frac{1}{2}$. Let $f^{[2004]}(x)$ be equal to $y$. Thus $f(y)=\frac{1}{2}$ or $y=\frac{1}{4}$ or $\frac{3}{4}$. Now we know $f^{[2004]}(x)$ is equal to $\frac{1}{4}$ or $\frac{3}{4}$. Now we know that $f(f^{[2003]}(x))=\frac{1}{4}$ or $\frac{3}{4}$. Now we solve for $f^{[2003]}(x)$ and let $f^{[2003]}(x)=z$. Thus $f(z)$ is equal to $\frac{1}{8}$,$\frac{7}{8}$,$\frac{5}{8}$,and $\frac{3}{8}$. As we see, $f^{[2005]}(x)$ has 1 solution, $f^{[2004]}(x)$ has 2 solutions, and $f^{[2003]}(x)$ has 4 solutions. Thus for each iteration we double the number of possible solutions. There are $2005$ iterations and thus the number of solutions is $2^{2005}$ $\Rightarrow\boxed{E}$

See Also

2005 AMC 12A (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 12 Problems and Solutions

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