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avatar+26400 
+13

$$\mathbf{12\binom{3}{3} + 11\binom{4}{3} + 10\binom{5}{3} + \cdots + 2\binom{13}{3} +\binom{14}{3} = \;?}$$

 

$$\small{\text{$
\begin{array}{l}
\mathbf{
12\binom{3}{3} +
11\binom{4}{3} +
10\binom{5}{3} +
9\binom{6}{3} +
8\binom{7}{3} +
7\binom{8}{3} +
6\binom{9}{3} +
5\binom{10}{3} +
4\binom{11}{3} +
3\binom{12}{3} +
2\binom{13}{3} +
1\binom{14}{3} } \\ \\
=
\mathbf{
\binom{4}{4} +
\binom{5}{4} +
\binom{6}{4} +
\binom{7}{4} +
\binom{8}{4} +
\binom{9}{4} +
\binom{10}{4} +
\binom{11}{4} +
\binom{12}{4} +
\binom{13}{4} +
\binom{14}{4} +
\binom{15}{4}
}\\\\
=
\mathbf{
\binom{16}{5}
}\\\\
=
\mathbf{
\dfrac{16!}{5!\cdot 11!}
}\\\\
=
\mathbf{
\dfrac{12\cdot 13\cdot 14\cdot 15\cdot 16}{5!}
}\\\\
=
\mathbf{
\dfrac{12\cdot 13\cdot 14\cdot 15\cdot 16}{1\cdot 2\cdot 3\cdot 4\cdot 5}
}\\\\
=
\mathbf{
\left(\dfrac{12}{3}\right)\cdot 13 \cdot
\left(\dfrac{14}{2}\right)\cdot
\left(\dfrac{15}{5}\right)\cdot
\left(\dfrac{16}{4}\right)
}\\\\
=
\mathbf{4 \cdot 13 \cdot 7 \cdot 3 \cdot 4}\\\\
=
\mathbf{13 \cdot 16 \cdot 21}\\\\
=
\mathbf{4368}
\end{array}
$}}$$

 

see Hockey Stick Pattern: http://ptri1.tripod.com/

If a diagonal of numbers of any length is selected starting at any of the 1's bordering the sides of the triangle and ending on any number inside the triangle on that diagonal, the sum of the numbers inside the selection is equal to the number below the end of the selection that is not on the same diagonal itself. If you don't understand that, look at the drawing.
1+6+21+56 = 84
1+7+28+84+210+462+924 = 1716
1+12 = 13

Apr 30, 2015
 #1
avatar+26400 
+10

$$\[\binom50+\binom51+\binom62+\binom71+\binom83+\binom92+\binom{10}4+\binom{11}3\] = 495$$

see: http://web2.0calc.com/#binom(5,0)+binom(5,1)+binom(6,2)+binom(7,1)+binom(8,3)+binom(9,2)+binom(10,4)+binom(11,3) and push the "=" Button

 

$$\small{\text{
$
\begin{array}{cccccccccccccccccccccccccc}
&&&&&&&&&&&\begin{pmatrix} 0\\0\end{pmatrix}=1\\
&&&&&&&&&&\begin{pmatrix} 1\\0\end{pmatrix}=1&&
\begin{pmatrix} 1\\1\end{pmatrix}=1\\
&&&&&&&&&\begin{pmatrix} 2\\0\end{pmatrix}=1&
&\begin{pmatrix} 2\\1\end{pmatrix}=2&
&\begin{pmatrix} 2\\2\end{pmatrix}=1\\
&&&&&&&&\begin{pmatrix} 3\\0\end{pmatrix}=1&&
\begin{pmatrix} 3\\1\end{pmatrix}=3&&
\begin{pmatrix} 3\\2\end{pmatrix}=3&&
\begin{pmatrix} 3\\3\end{pmatrix}=1\\
&&&&&&&\begin{pmatrix} 4\\0\end{pmatrix}=1&
&\begin{pmatrix} 4\\1\end{pmatrix}=4&
&\begin{pmatrix} 4\\2\end{pmatrix}=6&
&\begin{pmatrix} 4\\3\end{pmatrix}=4&
&\begin{pmatrix} 4\\4\end{pmatrix}=1\\
&&&&&&\begin{pmatrix} 5\\0\end{pmatrix}=1&&
\begin{pmatrix} 5\\1\end{pmatrix}=5&&
\begin{pmatrix} 5\\2\end{pmatrix}=10&&
\begin{pmatrix} 5\\3\end{pmatrix}=10&&
\begin{pmatrix} 5\\4\end{pmatrix}=5&&
\begin{pmatrix} 5\\5\end{pmatrix}=1\\
&&&&&\begin{pmatrix} 6\\0\end{pmatrix}=1&
&\begin{pmatrix} 6\\1\end{pmatrix}=6&
&\begin{pmatrix} 6\\2\end{pmatrix}=15&
&\begin{pmatrix} 6\\3\end{pmatrix}=20&
&\begin{pmatrix} 6\\4\end{pmatrix}=15&
&\begin{pmatrix} 6\\5\end{pmatrix}=6&
&\begin{pmatrix} 6\\6\end{pmatrix}=1\\
&&&&\begin{pmatrix} 7\\0\end{pmatrix}=1&&
\begin{pmatrix} 7\\1\end{pmatrix}=7&&
\begin{pmatrix} 7\\2\end{pmatrix}=21&&
\begin{pmatrix} 7\\3\end{pmatrix}=35&&
\begin{pmatrix} 7\\4\end{pmatrix}=35&&
\begin{pmatrix} 7\\5\end{pmatrix}=21&&
\begin{pmatrix} 7\\6\end{pmatrix}=7&&
\begin{pmatrix} 7\\7\end{pmatrix}=1\\
&&&\begin{pmatrix} 8\\0\end{pmatrix}=1&
&\begin{pmatrix} 8\\1\end{pmatrix}=8&
&\begin{pmatrix} 8\\2\end{pmatrix}=28&
&\begin{pmatrix} 8\\3\end{pmatrix}=56&
&\begin{pmatrix} 8\\4\end{pmatrix}=70&
&\begin{pmatrix} 8\\5\end{pmatrix}=56&
&\begin{pmatrix} 8\\6\end{pmatrix}=28&
&\begin{pmatrix} 8\\7\end{pmatrix}=8&
&\begin{pmatrix} 8\\8\end{pmatrix}=1\\
&&\begin{pmatrix} 9\\0\end{pmatrix}=1&&
\begin{pmatrix} 9\\1\end{pmatrix}=9&&
\begin{pmatrix} 9\\2\end{pmatrix}=36&&
\begin{pmatrix} 9\\3\end{pmatrix}=84&&
\begin{pmatrix} 9\\4\end{pmatrix}=126&&
\begin{pmatrix} 9\\5\end{pmatrix}=126&&
\begin{pmatrix} 9\\6\end{pmatrix}=84&&
\begin{pmatrix} 9\\7\end{pmatrix}=36&&
\begin{pmatrix} 9\\8\end{pmatrix}=9&&
\begin{pmatrix} 9\\9\end{pmatrix}=1\\
&\begin{pmatrix} 10\\0\end{pmatrix}=1&
&\begin{pmatrix} 10\\1\end{pmatrix}=10&
&\begin{pmatrix} 10\\2\end{pmatrix}=45&
&\begin{pmatrix} 10\\3\end{pmatrix}=120&
&\begin{pmatrix} 10\\4\end{pmatrix}=210&
&\begin{pmatrix} 10\\5\end{pmatrix}=252&
&\begin{pmatrix} 10\\6\end{pmatrix}=210&
&\begin{pmatrix} 10\\7\end{pmatrix}=120&
&\begin{pmatrix} 10\\8\end{pmatrix}=45&
&\begin{pmatrix} 10\\9\end{pmatrix}=10&
&\begin{pmatrix} 10\\10\end{pmatrix}=1\\
\begin{pmatrix} 11\\0\end{pmatrix}=1&&
\begin{pmatrix} 11\\1\end{pmatrix}=11&&
\begin{pmatrix} 11\\2\end{pmatrix}=55&&
\begin{pmatrix} 11\\3\end{pmatrix}=165&&
\begin{pmatrix} 11\\4\end{pmatrix}=330&&
\begin{pmatrix} 11\\5\end{pmatrix}=462&&
\begin{pmatrix} 11\\6\end{pmatrix}=462&&
\begin{pmatrix} 11\\7\end{pmatrix}=330&&
\begin{pmatrix} 11\\8\end{pmatrix}=165&&
\begin{pmatrix} 11\\9\end{pmatrix}=55&&
\begin{pmatrix} 11\\10\end{pmatrix}=11&&
\begin{pmatrix} 11\\11\end{pmatrix}=1
\end{array}
$}}$$

 

$$\small{\text{
$
\binom50+\binom51+\binom62+\binom71+\binom83+\binom92+\binom{10}4+\binom{11}3= 1+5+15+7+56+36+210+165 = 495
$}}$$

.
Apr 30, 2015
 #1
avatar+190 
0
Apr 30, 2015
 #1
avatar+130511 
+12

Look at the following graph, Mellie........https://www.desmos.com/calculator/9p3hhbrwl2

 

The graph of all the  possible arrival times - in minutes after 5 PM - by both Alice and Bob is bounded by x = 0, y = 0 x= 60 and y = 60.

 

Let the x values in the graph be the number of minutes after 5 PM that Alice arrives at the party. And let the y values be the number of minutes after 5 PM that Bob arrives at the party.  For example, at (0,0), both arrive at 5PM and at (60,60), both arrive at 6 PM.   At (0, 45)....Alice arrives at 5PM and Bob arrives at 5:45 PM. At (22.5, 22.5), both arrive at 5:22:30. And at (45, 0), Alice arrives 45 minutes after 5 PM and Bob arrives exactly at 5 PM.

 

But, the times we are interested in lie beneath the graph of x + y  ≤ 45.

 

And this area is bounded by x = 0, y = 0 and x + y  ≤ 45 . And it equals  45^2 / 2  = 1012.5 sq units

 

Note that the area of the total  possible arrival times   = 60 x 60  = 3600 sq units

 

So....the probabilty that the sum of  Alice's and Bob's arrival times after 5PM are less than 45 minutes = 

1012.5 / 3600  = 9/32

 

 

  

Apr 30, 2015
 #1
avatar+3502 
0
Apr 30, 2015

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