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(a) Let's call (a,b,c) a smiley face if b is less than a and b is less than c, because when we plot the graph, we get a happy face!  And if b is greater than a and b is greater than c, that's a frowny face, because we get a frowny face when we turn a smiely face up-side-down.

 

There are other kinds of faces like smirks (like a is less than b and b is less than c) and neutral faces (like when a is equal to b and b is equal to c).  If the face is neutral, then a equals b and b equals c, so when we choose a, b, and c are also chosen, and there are n choices for a, so there are n neutral faces.

 

Now we count the number of smirks.  There are n ways to choose a, and there are n - 1 ways to choose b.  We also multiply by 3, because the value that we chose for a could have also been the value of b, or the value of c.  So there are 3n(n - 1) smirks.

 

Now we count the number of smiley faces.  There are n ways to choose a, then n - 1 ways to choose b, then n - 2 ways to choose c.  So there are n(n - 1)(n - 2) = 3C(n,3) smiley faces.  By symmetry, there are 3C(n,3) frowny faces.

Therefore, the total number of faces is n^3 = n + 3n(n - 1) + 6C(n,3).

 

(b) To choose three numbers, we can choose two groups one group with two numbers and the other group has one number.  The total of n + 2 numbers can be separated into two groups.

 

In the first way, one group has 2 numbers, and the other group has n numbers.  They form a total of n + 2 numbers.  There are C(2,2) = 1 ways to choose two numbers from the 2 group.  There are C(n,1) = n ways to choose one number from the one group.  This gives us a first term of 1*n.

 

In the second way, one group has three numbers, and the other group has n - 1 numbers.  They form a total of n - 1 numbers.  There are C(3,2) = 2 ways to choose two numbers from the 2 group.  There are C(n - 1,1) = n - 1 ways to choose one number from the one group.  This gives us a second term of 2*(n - 1).  The pattern will continue until we reach n.  So the two sides are equal.

Feb 13, 2020
 #1
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The trapezoid is divided into 5 parts horizontally with AB and CD are equally divided.
The Trapezoid also divided into 5 parts vertically with AD and BC are devided equally.
If the shaded regions K has area 35 and the shaded region L has area 55.
Determine the area of the trapezoid ABCD.

 

\(\begin{array}{|rcll|} \hline 5b+2*\frac{1}{5}x = 5a &\text{ or }& \boxed{b = a-\frac{2}{25}x} \\ 5c+2*\frac{2}{5}x = 5a &\text{ or }& \boxed{c = a-\frac{4}{25}x} \\ 5d+2*\frac{3}{5}x = 5a &\text{ or }& \boxed{d = a-\frac{6}{25}x} \\ 5e+2*\frac{4}{5}x = 5a &\text{ or }& \boxed{e = a-\frac{8}{25}x} \\ 5f+2x = 5a &\text{ or }& \boxed{f = a-\frac{2}{5}x} \\ \hline \end{array}\)

 

\(\begin{array}{|rcll|} \hline \text{trapezoid } ABCD &=& \left(\dfrac{5a+5f}{2}\right)5h \quad | \quad \boxed{f = a-\frac{2}{5}x} \\ &=& \left(\dfrac{5a+5(a-\frac{2}{5}x)}{2}\right)5h \\ &=& \left(\dfrac{10a-2x}{2}\right)5h \\ &=& (5a-x)5h \\ \mathbf{\text{trapezoid } ABCD} &=& \mathbf{25ah-5xh} \\ \hline \end{array}\)

 

\(\begin{array}{|rcll|} \hline \mathbf{K:} \\ \hline 35 &=& \left(\frac{d+e}{2}\right)h + 2 \left( \frac{e+f}{2}\right)h \\ 35 &=& \left(\frac{d+e}{2}\right)h + ( e+f )h \\ 35 &=& \left(\frac{a-\frac{6}{25}x+a-\frac{8}{25}x}{2}\right)h + ( a-\frac{8}{25}x+a-\frac{2}{5}x )h \\ \ldots \\ \mathbf{35} &=& \mathbf{3ah-xh} \quad (1) \\ \hline \end{array} \)

 

\(\begin{array}{|rcll|} \hline \mathbf{L:} \\ \hline 55 &=& 2 \left( \frac{a+b}{2}\right)h + \left(\frac{b+c}{2}\right)h \\ 55 &=& ( a+b )h + \left(\frac{b+c}{2}\right)h \\ 55 &=& ( a+a-\frac{2}{25}x )h + \left( \frac{ a-\frac{2}{25}x + a-\frac{4}{25}x }{2} \right)h \\ \ldots \\ \mathbf{55} &=& \mathbf{3ah-\frac{5}{25}xh} \quad (2) \\ \hline \end{array} \)

 

\(\begin{array}{|lrcll|} \hline (2)-(1): & 3ah-\frac{5}{25}xh -(3ah-xh) &=& 55-35 \\ & 3ah-\frac{5}{25}xh -3ah+ xh &=& 20 \\ & -\frac{5}{25}xh + xh &=& 20 \\ & \frac{20}{25}xh &=& 20 \\ & \frac{xh}{25} &=& 1 \\ & \mathbf{xh} &=& \mathbf{25} \\\\ (1): & 3ah &=& 35+xh \\ & 3ah &=& 35+25 \\ & 3ah &=& 60 \\ & \mathbf{ah} &=& \mathbf{20} \\ \hline \end{array}\)

 

\(\begin{array}{|rcll|} \hline \mathbf{\text{trapezoid } ABCD} &=& \mathbf{25ah-5xh} \\ &=& 25*20-5*25 \\ &=& 500-125 \\ \mathbf{\text{trapezoid } ABCD} &=& \mathbf{375} \\ \hline \end{array} \)

 

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Feb 13, 2020
 #5
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Feb 13, 2020

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