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2008 iTest Problems/Problem 64 - Revision history
2024-03-28T09:53:32Z
Revision history for this page on the wiki
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https://artofproblemsolving.com/wiki/index.php?title=2008_iTest_Problems/Problem_64&diff=172325&oldid=prev
Sigmapie at 01:31, 5 March 2022
2022-03-05T01:31:29Z
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<td colspan="2" style="background-color: #fff; color: #222; text-align: center;">Revision as of 01:31, 5 March 2022</td>
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<tr><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>~sigma</div></td><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>~sigma</div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Note: Recursive binomial expansion is just a fancy way of generalizing a pattern of recursive functions of the form <math>f(n)=f(n-k)+f(n-k-1)</math>.  </div></td><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Note: Recursive binomial expansion is just a fancy way of generalizing a pattern of recursive functions of the form <math>f(n)=f(n-k)+f(n-k-1)</math>.  </div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>==See Also==</div></td><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>==See Also==</div></td></tr>
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Sigmapie
https://artofproblemsolving.com/wiki/index.php?title=2008_iTest_Problems/Problem_64&diff=172324&oldid=prev
Sigmapie at 01:30, 5 March 2022
2022-03-05T01:30:43Z
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<td colspan="2" style="background-color: #fff; color: #222; text-align: center;">Revision as of 01:30, 5 March 2022</td>
</tr><tr><td colspan="2" class="diff-lineno" id="mw-diff-left-l17" >Line 17:</td>
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<tr><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Since the twins drew all the possible rows, there are <math>7+20+1 = \boxed{28}</math> rows that the twins drew.</div></td><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>Since the twins drew all the possible rows, there are <math>7+20+1 = \boxed{28}</math> rows that the twins drew.</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">==Solution 2 (Recursion)==</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">Let <math>f(n)</math> be the amount of valid ways to arrange pairs and trios of symbols. Note that this is equal to <math>f(n-2)+f(n-3)</math>, because both <math>n-2</math> and <math>n-3</math> conveniently allow us to add a final pair or trio to get <math>n</math> symbols. Using binomial recursive expansion, we obtain <math>f(15) = f(3)+4f(4)+6f(5)+4f(6)+f(7)</math>. Since <math>f(2) = f(3) = f(4) = 1</math>, this can be simplified further to <math>1+4+6(1+1)+4(1+1)+(1+2) = \boxed{28}</math>. </ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;"></ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">~sigma</ins></div></td></tr>
<tr><td colspan="2"> </td><td class='diff-marker'>+</td><td style="color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #a3d3ff; vertical-align: top; white-space: pre-wrap;"><div><ins style="font-weight: bold; text-decoration: none;">Note: Recursive binomial expansion is just a fancy way of generalizing a pattern of recursive functions of the form <math>f(n)=f(n-k)+f(n-k-1)</math>. </ins></div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>==See Also==</div></td><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>==See Also==</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>{{2008 iTest box|num-b=63|num-a=65}}</div></td><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>{{2008 iTest box|num-b=63|num-a=65}}</div></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"></td></tr>
<tr><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>[[Category:Introductory Combinatorics Problems]]</div></td><td class='diff-marker'> </td><td style="background-color: #f8f9fa; color: #222; font-size: 88%; border-style: solid; border-width: 1px 1px 1px 4px; border-radius: 0.33em; border-color: #eaecf0; vertical-align: top; white-space: pre-wrap;"><div>[[Category:Introductory Combinatorics Problems]]</div></td></tr>
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Sigmapie
https://artofproblemsolving.com/wiki/index.php?title=2008_iTest_Problems/Problem_64&diff=96868&oldid=prev
Rockmanex3: Solution to Problem 64
2018-08-08T12:58:50Z
<p>Solution to Problem 64</p>
<p><b>New page</b></p><div>==Problem==<br />
<br />
Alexis and Joshua are walking along the beach when they decide to draw symbols in the sand. Alex draws only stars and only draws them in pairs while Joshua draws only squares in trios. "Let's see how many rows of <math>15</math> adjacent symbols we can make this way," suggests Josh. Alexis is game for the task and the two get busy drawing. Some of their rows look like <br />
<br />
<cmath>\begin{array}{ccccccccccccccc}\vspace{10pt}*&*&*&*&*&*&*&*&*&*&*&*&\blacksquare&\blacksquare&\blacksquare\\\vspace{10pt}\blacksquare&\blacksquare&\blacksquare&*&*&*&*&*&*&*&*&*&*&*&*\\\vspace{10pt}\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&*&*&*&*&*&*&\blacksquare&\blacksquare&\blacksquare\\\vspace{10pt}\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare&\blacksquare\\\vspace{10pt}*&*&*&*&*&*&\blacksquare&\blacksquare&\blacksquare&*&*&*&*&*&*\end{array} </cmath><br />
<br />
The twins decide to count each of the first two rows above as distinct rows, even though one is the mirror image of the other. But the last of the rows above is its own mirror image, so they count it only once. Around an hour later, the twins realized that they had drawn every possible row exactly once using their rules of stars in pairs and squares in trips. How many rows did they draw in the sand? <br />
<br />
==Solution==<br />
<br />
We will use [[casework]] on the number of triplets drawn in order to count the number of rows drawn.<br />
<br />
* If <math>1</math> triplet is drawn, then there are <math>6</math> pairs needed. There is a total of <math>\binom{7}{1} = 7</math> rows in this case.<br />
* If <math>3</math> triplets are drawn, then there are <math>3</math> pairs needed. There is a total of <math>\binom{6}{3} = 20</math> rows in this case.<br />
* If <math>5</math> triplets are drawn, then there are <math>0</math> pairs needed. There is only <math>1</math> row in this case.<br />
<br />
Since the twins drew all the possible rows, there are <math>7+20+1 = \boxed{28}</math> rows that the twins drew.<br />
<br />
==See Also==<br />
{{2008 iTest box|num-b=63|num-a=65}}<br />
<br />
[[Category:Introductory Combinatorics Problems]]</div>
Rockmanex3