## Tuesday, February 14, 2017

### Hints/links or Solutions to 2014 Harder Mathcounts State Sprint and Target question

Links, notes, Hints or/and solutions to 2014 Mathcounts state harder problems.
2014, 2015 Mathcounts state are harder

Sprint round:

#14 :
Solution I :
(7 + 8 + 9)  + (x + y + z)  is divisible by 9, so the sum of the three variables could be 3, 12, or 21.
789120 (sum of 3 for the last three digits) works for 8 but not for 7.
21 is too big to distribute among x, y and z (all numbers are district),
thus only x + y + z = 12 works and z is an even number
__ __ 0 does't work (can't have 6 6 0 and the other pairs all have 7, 8 or 9)
264 works (789264 is the number)

Solution II :
789000 divided by the LCM of 7, 8 and 9, which is 504 = 1565.47...
Try 504 * 1566 = 789264 (it works)

#18:
Watch this video from Mathcounts mini and use the same method for the first question,
you'll be able to get the answer. It's still tricky, though.

#23 : Drop the heights of the two isosceles triangles and use similar triangles to get the length of FC.
Then solve.

#24:
The key is to see 210 is 1024 or about 103

#25:
As you can see, there are two Pythagorean Triples : 9-12-15 and 9-40-41.
Base (40-12) = 28 gives you the smallest area.
The answer is 28 * 18 = 504

#26 : Let there be A, B, C three winners. There are 4 cases to distribute the prizes.
A     B    C
1      1     5    There are 7C1 * 6C1 * $$\dfrac {3!} {2!}$$ = 126 ways -- [you can skip the last part for C
because it's 5C5 = 1]

1       2    4    There are 7C1* 6C2 * 3! = 630

1      3     3    There are 7C1 * 6C3 * $$\dfrac {3!} {2!}$$ = 420

2      2    3     There are 7C2 * 5C2 * 3 (same as above)

If you can't see why it's $$\dfrac {3!} {2!}$$ when there is one repeat, try using easier case to help you understand.

What about A, B two winners and 4 prizes ?
There are 2 cases, 1 3 or 2 2, and you'll see how it's done.

#27 : Read this and you'll be able to solve this question at ease, just be careful with the sign change.
Vieta's Formula and the Identity Theory

#28: There are various methods to solve this question.
I use binomial expansion :
$$11^{12}=\left( 13-2\right)^{12}=12C0*13^{12}$$+ $$12C1*13^{11}*2^{1}$$+... $$12C11*13^{1}*2^{11}$$+ $$12C12*2^{12}$$ Most of the terms will be evenly divided by 13 except the last term, which is $$2^{12}$$ or 4096, which, when divided by 13, leaves a remainder of 1.

Solution II :
$$11\equiv -2\left ( mod13\right)$$ ; $$(-2)^{12}\equiv 4096\equiv 1\left ( mod13\right)$$

Solution III :
Or use Fermat's Little Theorem (Thanks, Spencer !!)
$$11^{13-1}\equiv 11^{12}\equiv1 (mod 13)$$

Target Round :

#3: Lune of Hippocrates : in seconds solved question.
^__^

#6: This question is very similar to this Mathcounts Mini.
My students should get a virtual bump if they got this question wrong.

#8: Solution I : by TMM (Thanks a bunch !!)
Using similar triangles and Pythagorean Theorem.

The height of the cone, which can be found usinthe Pythagorean  is $\sqrt{10^2-5^2}=5\sqrt{3}$.
Usingthediagram below, let $r$ be the radius of the top cone and let $h$ be the height of the topcone.
Let $s=\sqrt{r^2+h^2}$ be the slant height of the top cone.

Drawing the radius as shown in the diagram, we have two right triangles. Since the bases of the top cone and the original cone are parallel, the two right triangles are similar. So we have the proportion$$\dfrac{r}{5}=\dfrac{s}{10}=\dfrac{\sqrt{r^2+h^2}}{10}.$$Cross multiplying yields $$10r=5\sqrt{r^2+h^2}\implies 100r^2=25r^2+25h^2\implies 75r^2=25h^2\implies 3r^2=h^2\implies h=r\sqrt{3}.$$This is what we need.

Next, the volume of the original cone is simply $\dfrac{\pi\times 25\times 5\sqrt{3}}{3}=\dfrac{125\sqrt{3}}{3}$.

The volume of the top cone is $\dfrac{\pi\times r^2h}{3}$.
From the given information, we know that $$\dfrac{125\sqrt{3}}{3}-\dfrac{\pi\times r^2h}{3}=\dfrac{125\sqrt{3}}{9}\implies 125\sqrt{3}-r^2h=\dfrac{125\sqrt{3}}{3}\implies r^2h=\dfrac{250\sqrt{3}}{3}.$$We simply substitute the value of $h=r\sqrt{3}$ from above to yield $$r^3\sqrt{3}=\dfrac{250\sqrt{3}}{3}\implies r=\sqrt[3]{\frac{250}{3}}.$$We will leave it as is for now so the decimals don't get messy.

We get $h=r\sqrt{3}\approx 7.56543$ and $s=\sqrt{r^2+h^2}\approx 8.7358$.

The lateral surface area of the frustum is equal to the lateral surface area of the original cone minus the lateral surface area of the top cone. The surface area of the original cone is simply
$5\times 10\times \pi=50\pi$.
The surface area of the top cone is $\pi\times r\times s\approx 119.874$.
So our lateral surface area is

All we have left is to add the two bases. The total area of thebases is $25\pi+\pi\cdot r^2\approx 138.477$. So our final answer is $$37.207+138.477=175.684\approx\boxed{176}.$$
Solution II
Using dimensional change and ratio, proportion.

Cut the cone and observe the shape.

The circumference of the larger circle is 20pi (10 is the radius) and the base of

the cone circle circumference is 10pi (5 is the radius), which means that the cut-off cone shape is a half circle because it's $$\dfrac {10\pi } {20\pi }$$ or $$\dfrac {1 } {2 }$$ of the larger circle. (180 degrees)

To find the part that is the area of the frustum not including the top and bottom circles,

you use the area of the half circle minus the area of the smaller half circle.

Since the volume ratio of the smaller cone to larger cone = 2 to 3, the side ratio of the

two radius is $$\dfrac {\sqrt [3] {2}} {\sqrt [3] {3}}$$.

Using this ratio, we can get the radius of the smaller circle as 10 * $$\dfrac {\sqrt [3] {2}} {\sqrt [3] {3}}$$ and the radius of the top circle of the frustum as 5 * $$\dfrac {\sqrt [3] {2}} {\sqrt [3] {3}}$$.

Now we can solve this :

$$\dfrac {1 } {2 }$$$$\left[ 10^{2}\pi -\left( 10\times \dfrac {\sqrt [3] {2}} {\sqrt [3] {3}}\right) ^{2}\pi \right]$$ + $$5^{2}\pi +\left( 5\times \dfrac {\sqrt [3] {2}} {\sqrt {3}}\right) ^{2}\pi$$ = about 176 (after you round up)ional change and ratio, proportion.

Cut the cone and observe the shape.

The circumference of the larger circle is 20pi (10 is the radius) and the base of

the cone circle circumference is 10pi (5 is the radius), which means that the cut-off cone shape is a half circle because it's $$\dfrac {10\pi } {20\pi }$$ or $$\dfrac {1 } {2 }$$ of the larger circle. (180 degrees)

To find the part that is the area of the frustum not including the top and bottom circles,

you use the area of the half circle minus the area of the smaller half circle.

Since the volume ratio of the smaller cone to larger cone = 2 to 3, the side ratio of the

two radius is $$\dfrac {\sqrt [3] {2}} {\sqrt [3] {3}}$$.

Using this ratio, we can get the radius of the smaller circle as 10 * $$\dfrac {\sqrt [3] {2}} {\sqrt [3] {3}}$$ and the radius of the top circle of the frustum as 5 * $$\dfrac {\sqrt [3] {2}} {\sqrt [3] {3}}$$.

Now we can solve this :

$$\dfrac {1 } {2 }$$$$\left[ 10^{2}\pi -\left( 10\times \dfrac {\sqrt [3] {2}} {\sqrt [3] {3}}\right) ^{2}\pi \right]$$ + $$5^{2}\pi +\left( 5\times \dfrac {\sqrt [3] {2}} {\sqrt {3}}\right) ^{2}\pi$$ = about 176 (after you round up)

Solution III : Another way to find the surface area of the Frustum is :
median of the two half circle [same as median of the two bases] * the height [difference of the two radius]
$$\dfrac {1} {2}\left( 2\times 10\pi + 2\times 10\times \dfrac {\sqrt [3] {2}} {\sqrt [3] {3}}\pi \right)$$* $$\left( 10-10\times \dfrac {\sqrt [3] {2}} {\sqrt [3]{3}}\right)$$

### 2013 Mathcounts State Harder Problems

Trickier 2013 Mathcounts State Sprint Round questions :
Sprint #14:
From Varun:
Assume the term "everything" refers to all terms in the given set.
1 is a divisor of everything, so it must be first.
Everything is a divisor of 12, so it must be last.
The remaining numbers left are 2, 3, 4, and 6.
2 and 3 must come before 6, and 2 must come before 4.
Therefore, we can list out the possibilities for the middle four digits:
2,3,4,6
2,3,6,4
3,2,4,6
3,2,6,4
2,4,3,6
There are 5 ways--therefore 5 is the answer.

From Vinjai:
Here's how I did #14:
First, notice that 1 must be the first element of the set and 12 must be the last one.
So that leaves only 2,3,4,6 to arrange.
We can quickly list them out.
The restrictions are that 2 must be before 4, 3 must be before 6, and 2 must be before 6:
2,3,4,6          3,2,4,6          4,2,3,6            6,2,3,4
2,3,6,4          3,2,6,4          4,2,6,3            6,2,4,3
2,4,3,6          3,4,2,6          4,3,2,6            6,3,2,4
2,4,6,3          3,4,6,2          4,3,6,2            6,3,4,2
2,6,3,4          3,6,2,4          4,6,2,3            6,4,2,3
2,6,4,3          3,6,4,2          4,6,3,2            6,4,3,2

Only the bold ones work. So, the answer is 5.

#17: Common dimensional change problem
$$\overline {ZY}:\overline {WV}=5:8$$ -- line ratio
The volume ratio of the smaller cone to the larger cone is thus $$5^{3}: 8^{3}$$.
The volume of the frustum is the volume of the larger cone minus the volume of the smaller cone
= $$\dfrac {8^{3}-5^{3}} {8^{3}}\times \dfrac {1} {3}\times 8^{2}\times 32\times \pi$$ = 516$$\pi$$

More problems to practice from Mathcounts Mini

#24:  The answer is $$\dfrac {1} {21}$$.

#28: Hats off to students who can get this in time !! Wow!!
From Vinjai:

For #28, there might be a nicer way but here's how I did it when I took the sprint round:

# 4's     # 3's     # 2's       # 1's     # ways
1         2          0            0          3
1         1          1            1          24
1         1          0            3          20
1         0          3            0          4
1         0          2            2          30
1         0          1            4          30
0         2          2            0          6
0         2          1            2          30
0         2          0            4          15
0         1          3            1          20
0         1          2            3          60
0         0          3            4          35

TOTAL: 277

#29:
$$\Delta ADE$$ is similar to $$\Delta ABC$$
Let the two sides of the rectangle be x and y (see image on the left)

$$\dfrac {x} {21}=\dfrac {8-y} {8}$$
x =$$\dfrac {21\left( 8-y\right) } {8}$$

xy =  $$\dfrac {21\left( 8-y\right) } {8}$$  * y = $$\dfrac {-21y\left( y-8\right) } {8}$$ =
$$\dfrac {-2l\left( y-4\right) ^{2}+21\times 16} {8}$$

From the previous equation you know when y = 4, the area $$\dfrac {21\times 16} {8}$$is the largest. The answer is 42.

Here is a proof to demonstrate that the largest area of a rectangle inscribed in a triangle is
half of the area of that triangle.

#30:
Solution I :
If (x, y) are the coordinates of the center of rotational points, it will be equal distance from A and A' as well as from B and B'.
Use distance formula, consolidate/simplify and solve the two equations, you'll get the answer (4, 1).

Solution II:

How to find the center of rotation from Youtube.

From AoPS using the same question

To sum up:
First, connect the corresponding points, in this case A to A' and B to B'.
Second, find the equation of the perpendicular bisector of line $$\overline {AA'}$$, which is
y =  - x + 5
and $$\overline {BB'}$$, which is y = 5x - 19
The interception of the two lines is the center of rotation.

2013 Mathcounts Target :
#3:
RT = D, unit conversions and different rates are tested here:

Make Joy's rate (speed) uphill be x m/s, his downhill speed be 2x m/s.

It takes Greg 3000 seconds (time) to reach the starting point and that is also what it takes Joy to
ride up to the hill and down to the same point.

$$\dfrac {7000} {x}+\dfrac {10000} {2x}=3000$$ $$\rightarrow$$ x = 4 m/s

#8:

Using "finding the height to the hypotenuse".( click to review)

you get $$\overline {CD}=\dfrac {7\times 24} {25}$$.

Using similar triangles ACB and ADC, you get  $$\overline {AD}=\dfrac {576} {25}$$.
[$$\dfrac {24} {x}=\dfrac {25} {24}$$]

Using angle bisector (click to review),

you have $$\overline {AC}:\overline {AD}=\overline {CE}:\overline {ED}= 24: \dfrac {576} {25}$$ = 600 : 576 = 25 : 24

$$\rightarrow$$$$\overline {ED}= \overline {CD}\times \dfrac {24} {24+25}$$ = $$\dfrac {7\times 24} {25}\times \dfrac {24} {24+25}$$ = $$\dfrac {576} {175}$$