219 lines
7.5 KiB
TeX
219 lines
7.5 KiB
TeX
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\section{\texttt{int}s and \texttt{float}s}
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\definition{}
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A \textit{signed 32-bit integer} (equivalently, a \texttt{long int}) consists of thirty-two binary digits, \par
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and is used represent a subset of the integers.
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\vspace{2mm}
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The first bit of a \texttt{long} tells us its sign:
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\begin{itemize}
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\item if the first bit of a \texttt{long} is \texttt{1}, it represents a negative number;
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\item if the first bit is \texttt{0}, it represents a positive number.
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\end{itemize}
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We do not need negative numbers today, so we will assume that the first bit is always zero. \par
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\note{If you'd like to know how negative integers are written, look up \say{two's complement} after class.}
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\vspace{2mm}
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We'll denote binary strings with the prefix \texttt{0b}. \par
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Underscores are added between every eight digits for readability, and have no meaning.
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\vspace{2mm}
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The value of a positive signed \texttt{long} is simply the value of its binary digits. \par
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For example:
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\begin{itemize}
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\item $\texttt{0b00000000\_00000000\_00000000\_00000000} = 0$
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\item $\texttt{0b00000000\_00000000\_00000000\_00000011} = 3$
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\item $\texttt{0b00000000\_00000000\_00000000\_00100000} = 32$
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\item $\texttt{0b00000000\_00000000\_00000000\_10000010} = 130$
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\end{itemize}
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Remember---we only need positive integers today. Assume the \say{sign} bit is always \texttt{0}.
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\problem{}
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What is the largest number that can be represented with a \texttt{long}?
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\begin{solution}
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$\texttt{0b01111111\_11111111\_11111111\_11111111} = 2^{31}$
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\end{solution}
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\vfill
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\problem{}
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What is the smallest possible number that can be represented with a \texttt{long}? \par
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\hint{
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You do not need to know \textit{how} negative numbers are represented. \par
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Assume that we do not skip any integers, and don't forget about zero.
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}
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\begin{solution}
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There are $2^{64}$ possible 32-bit patterns,
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of which 1 represents zero and $2^{31}$ represent positive numbers.
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\vspace{2mm}
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We therefore have access to $2^{64} - 1 - 2^{31}$ negative numbers,
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giving us a minimum representable value of $-2^{31} + 1$.
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\end{solution}
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\problem{}
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What is the value of the following longs?
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\begin{itemize}
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\item \texttt{0b00000000\_00000000\_00000101\_00111001}
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\item \texttt{0b00000000\_00000000\_00000001\_00101100}
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\item \texttt{0b00000000\_00000000\_00000100\_10110000}
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\end{itemize}
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\hint{The third conversion is easy---look carefully at the second.}
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\begin{solution}
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\begin{itemize}
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\item $\texttt{0b00000000\_00000000\_00000101\_00111001} = 1337$
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\item $\texttt{0b00000000\_00000000\_00000001\_00101100} = 300$
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\item $\texttt{0b00000000\_00000000\_00000010\_01011000} = 1200$
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\end{itemize}
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Notice that the third long is the second shifted left twice (i.e, multiplied by 4)
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\end{solution}
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\vfill
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\pagebreak
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\definition{}
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A \textit{signed 32-bit floating-point decimal} (equivalently, a \textit{float})
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consists of 32 binary digits, and is used to represent a subset of the real numbers.
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These 32 bits are interpreted as follows:
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\begin{center}
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\begin{tikzpicture}
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\node[anchor=south west] at (0, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (0.25, 0) {\texttt{\texttt{b}}};
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\node[anchor=south west] at (0.50, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (0.75, 0) {\texttt{\texttt{\_}}};
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\node[anchor=south west] at (1.00, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (1.25, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (1.50, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (1.75, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (2.00, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (2.25, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (2.50, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (2.75, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (3.00, 0) {\texttt{\texttt{\_}}};
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\node[anchor=south west] at (3.25, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (3.50, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (3.75, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (4.00, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (4.25, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (4.50, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (4.75, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (5.00, 0) {\texttt{\texttt{\_}}};
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\node[anchor=south west] at (5.25, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (5.50, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (5.75, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (6.00, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (6.25, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (6.50, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (6.75, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (7.00, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (7.25, 0) {\texttt{\texttt{\_}}};
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\node[anchor=south west] at (7.50, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (7.75, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (8.00, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (8.25, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (8.50, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (8.75, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (9.00, 0) {\texttt{\texttt{0}}};
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\node[anchor=south west] at (9.25, 0) {\texttt{\texttt{0}}};
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\draw (0.50, 0) -- (0.95, 0) node [midway, below=1mm] {sign};
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\draw (1.05, 0) -- (3.15, 0) node [midway, below=1mm] {exponent};
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\draw (3.30, 0) -- (9.70, 0) node [midway, below=1mm] {fraction};
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\end{tikzpicture}
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\end{center}
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In other words:
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\begin{itemize}[itemsep = 1mm]
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\item The first bit denotes the sign of the float's value. We'll label it $s$. \par
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If $s = 1$, this float is negative; if $s = 0$, it is positive.
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\item The next 8 bits represent the \textit{exponent} of this float. \par
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We'll call the value of these eight bits $E$. \par
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Naturally, $0 \leq E \leq 255$
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\item The remaining 23 bits represent the \textit{fraction} of this float. \par
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These 23 bits are interpreted as the fractional part of a binary decimal. \par
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For example, the bits \texttt{0b1010000\_00000000\_00000000} represents $0.5 + 0.125 = 0.625$.
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\end{itemize}
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\vspace{2mm}
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The final value of a float with sign $s$, exponent $E$, and fraction $F$ is
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\begin{equation*}
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(-1)^s ~\times~ 2^{E - 127} ~\times~ \left(1 + \frac{F}{2^{23}}\right)
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\end{equation*}
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Notice that this is very similar to decimal scientific notation, which is written as
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\begin{equation*}
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(\pm 1) ~\times~ 10^{e} ~\times~ (f)
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\end{equation*}
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\vfill
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\pagebreak
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\problem{}
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What is the value of \texttt{0b01000001\_10101000\_00000000\_00000000} if it is interpreted as a float? \par
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\hint{$21 \div 16 = 1.3125$}
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\begin{solution}
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This is 21:
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\begin{align*}
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&=~ 2^{131} \times \biggl(1 + \frac{2^{21} + 2^{19}}{2^{23}}\biggr) \\
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&=~ 2^{4} \times (1 + 0.25 + 0.0625) \\
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&=~ 16 \times (1.3125) \\
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&=~ 21
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\end{align*}
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\end{solution}
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\vfill
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\problem{}
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Encode $12.5$ as a float. \par
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\hint{$12.5 \div 8 = 1.5625$}
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\vspace{2mm}
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What is the value of the resulting 32 bits if they are interpreted as a long? \par
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\hint{A sum of powers of two is fine.}
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\begin{solution}
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\begin{align*}
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12.5
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&=~ 8 \times 1.5625 \\
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&=~ 2^{3} \times \biggl(1 + (0.5 + 0.0625)\biggr) \\
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&=~ 2^{130} \times \biggl(1 + \frac{2^{22} + 2^{19}}{2^{23}}\biggr)
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\end{align*}
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\linehack{}
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This is \texttt{0b01000001\_01001000\_00000000\_00000000}, \par
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which is $2^{30} + 2^{24} + 2^{22} + 2^{19} = 11,095,237,632$
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\end{solution}
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\vfill
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\pagebreak |