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%%% author: Ján Komara %%%
%%% email: komara@fmph.uniba.sk %%%
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\title{Vzorové príklady pre 1. domácu úlohu}
\author{Ján Komara}
%\date{}
\date{\today}
\maketitle
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\section*{1. príklad}
Uvažujme digitálne hodiny, ktoré sú nastavené na európsky 24-hodinový cyklus.
Tie ukazujú denný časový údaj vo formáte
\begin{center}
hodiny:minúty:sekundy.
\end{center}
Tak napríklad 01:24:58 je časový údaj, ktorý sa na takýchto hodinách môže vyskytnúť.
Spočítajme, koľkokrát za deň sa na hodinách vyskytne časový údaj
\begin{align*}
H_1 H_2 : M_1 M_2 : S_1 S_2
\end{align*}
taký, že
\begin{align*}
H_1 < H_2 < M_1 < M_2 < S_1 < S_2
.
\end{align*}
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\subsection*{Riešenie}
Najprv si všimnime, že prvý časový údaj s uvedenými vlastnosťami má tvar 01:23:45,
zatiaľ čo posledný takýto údaj má tvar 12:34:59.
Desiatky sekúnd $S_1$ sa musia vyberať spomedzi čísel $\{ 4, 5 \}$.
Pre každé také $S_1 \in \{ 4, 5 \}$ vyberieme ďalej pätice čísel
$H_1 H_2 M_1 M_2 S_2$ tak, aby platilo
\begin{align*}
0 \leq H_1 < H_2 < M_1 < M_2 < S_1 < S_2 \leq 9
.
\end{align*}
Ak $S_1 = 4$, tak počet možností takýchto výberov je rovný číslu 5,
pretože vtedy musí platiť
\begin{align*}
H_1 = 0 \land H_2 = 1 \land M_1 = 2 \land M_2 = 3 \land 5 \leq S_2 \leq 9
.
\end{align*}
Ak $S_1 = 5$, tak počet možností takýchto výberov je rovný číslu $5 \times 4$,
pretože vtedy musí platiť
\begin{align*}
0 \leq H_1 < H_2 < M_1 < M_2 \leq 4 \land 6 \leq S_2 \leq 9
.
\end{align*}
Hľadaný počet časových údajov je tak rovný číslu
\begin{align*}
5 + 5 \times 4 = 25
.
\end{align*}
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\section*{2. príklad}
Koľko $6$-ciferných čísel pozostávajúcich len z nenulových cifier
neobsahuje dve za sebou idúce tie isté cifry?
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\subsection*{Riešenie}
Podľa pravidla súčinu hľadaný počet je rovný číslu
\begin{align*}
9 \times 8 \times 8 \times 8 \times 8 \times 8 = 9 \times 8^5
.
\end{align*}
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