Pieces of the Puzzle

Algebra Level 5

f ( x ) = D H x 5 D 2 E H 2 x 4 + F G F x 3 C F D E F H x 2 + E H x D E 2 H 2 . f(x) = DHx^{5} - D^{2}EH^{2}x^{4} + FG^{F}x^{3} - C^{F}DEFHx^{2} + EHx - DE^{2}H^{2}.

N N is a real root of f ( x ) f(x) .


An analog clock is inscribed in an equilateral triangle of side length P P . The clock reads 12:45, and the hands of the clock enclose a circular sector of area Q Q . P P can be expressed as A B C D π A\sqrt{\frac{B\sqrt{C}}{Dπ}} , when simplified.


W X Y Z \overline{WXYZ} is a 4 4 -digit integer, whose digits add to a power of 2 2 and multiply to a power of 3 3 , with ( H D B ) (H-D-B) distinct prime factors. W X = Z Y = E \overline{WX}=\overline{ZY}=E and W X W≠X .


A regular hexagon of area Q Q is inscribed in the graph of y = x 2 y=x^{2} , touching the graph at four vertices. The distance from the center of the hexagon to the origin can be expressed as F G \frac{F}{G} when simplified.


J = A B J=A-B . There is a J x J JxJ grid and a D x D DxD grid. The J x J JxJ grid contains each integer from 1 1 to J 2 J^{2} inclusive. The D x D DxD grid contains each integer from 1 1 to D 2 D^{2} inclusive. A number is randomly chosen from the J x J JxJ grid; then, one is randomly chosen from the D x D DxD grid. H H % is the probability that the first number is not a factor of 234 234 and the second is a factor of 2340 2340 .


Find N N .

Details and Assumptions:

  • A , B , C , D , E , F , A, B, C, D, E, F, and G G are positive integers.

  • B B and D D are coprime.

  • F F and G G are coprime.

  • B , C , B, C, and D D are not divisible by any perfect square greater than 1 1 .

  • A program may be used to calculate roots of 5th degree polynomials.


The answer is 2015.

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1 solution

A A
Dec 11, 2015

Start with the information about the hexagon inscribed in a parabola; the area Q can be determined from the given information, rather than the other way around.

Draw the lines above to create three triangles. The side lengths of the equilateral triangles are equal to S S , the side lengths of the hexagon. Knowing the properties of 30 60 90 30-60-90 triangles, it is clear that the x x -coordinate of A A is S 2 \frac{S}{2} , and the height of each triangle is s 3 2 \frac { s\sqrt { 3 } }{ 2 } . Since points A and C lie on the parabola, their y y coordinates must equal the square of their x x -coordinates.

Using this, we can write a system of equations:

y A = ( S 2 ) 2 = S 2 4 { y }_{ A }={ \left( \frac { S }{ 2 } \right) }^{ 2 }=\frac { { S }^{ 2 } }{ 4 }

y A + S 3 2 = S 2 { y }_{ A }+\frac { S\sqrt { 3 } }{ 2 } ={ S }^{ 2 }

Substituting y A { y }_{ A } in the second equation:

S 2 4 + S 3 2 = S 2 3 S 2 2 3 S = 0 \frac { { S }^{ 2 } }{ 4 } +\frac { S\sqrt { 3 } }{ 2 } ={ S }^{ 2 } \Rightarrow 3{ S }^{ 2 }-2\sqrt { 3 } S=0

Since S 0 S\neq 0 ,

S = 2 3 S=\frac { 2 }{ \sqrt { 3 } }

Plugging the value of S into the first equation:

y A = 1 3 { y }_{ A }=\frac { 1 }{ 3 }

The y y -coordinate of the center of the hexagon (the distance between the center and the origin) is given by:

y H = y A + S 3 2 = F G = 4 3 { y }_{ H }{ =y }_{ A }+\frac { S\sqrt { 3 } }{ 2 } =\frac { F }{ G } =\frac { 4 }{ 3 }

We now know that F = 4 F=4 and G = 3 G=3 .

In addition, the area of the hexagon can be easily calculated:

Q = A p o t h e m × P e r i m e t e r 2 = S 3 2 × 6 S 2 = 2 3 = Q Q=\frac { Apothem\quad \times \quad Perimeter }{ 2 } =\frac { \frac { S\sqrt { 3 } }{ 2 } \times 6S }{ 2 } =2\sqrt { 3 } =Q

Now that Q is known, let's go to the part about the clock.

Since the hour is 45 60 = 3 4 \frac{45}{60}=\frac{3}{4} of the way complete, and the angle between 12 12 and 1 1 is 2 π 12 = π 6 \frac{2\pi}{12}=\frac{\pi}{6} :

θ = π 6 × 3 4 = π 8 \theta = \frac{\pi}{6}\times \frac{3}{4}=\frac{\pi}{8}

The angle of the total sector, then, is:

π 8 + π 2 = 5 π 8 \frac{\pi}{8}+\frac{\pi}{2}=\frac{5\pi}{8}

The radius of the clock can be found using the formula for the area of a circular sector, since the area Q is already known:

Q = 2 3 = 1 2 ( 5 π 8 ) ( r 2 ) r = 32 3 5 π Q=2\sqrt { 3 } =\frac { 1 }{ 2 } \left( \frac { 5\pi }{ 8 } \right) \left( { r }^{ 2 } \right) \Rightarrow r=\sqrt{\frac { 32\sqrt { 3 } }{ 5\pi } }

r is one leg of a 30 60 90 30-60-90 triangle, shown in the diagram. The other leg is half of P, the side length of the equilateral triangle. This fact can be used to calculate P:

P 2 = r 3 = 96 3 5 π = 4 6 3 5 π P = A B C D π = 8 6 3 5 π \frac { P }{ 2 } =r\sqrt { 3 } =\sqrt { \frac { 96\sqrt { 3 } }{ 5\pi } } =4\sqrt { \frac { 6\sqrt { 3 } }{ 5\pi } } \Rightarrow P=A\sqrt { \frac { B\sqrt { C } }{ D\pi } } =8\sqrt { \frac { 6\sqrt { 3 } }{ 5\pi } }

We now know that A = 8 A=8 , B = 6 B=6 , C = 3 C=3 , and D = 5 D=5 . This will be enough information to look at the part about grids.

The prime factorization of 234 234 is 2 × 3 2 × 13 2 \times 3^{2} \times 13 . Therefore, 1 1 , 2 2 , and 3 3 are factors of 234 234 , but 4 4 is not. There is a 1 4 \frac{1}{4} chance that the first number is not a factor of 234 234 .

The prime factorization of 2340 2340 is 2 2 × 3 2 × 5 × 13 2^{2} \times 3^{2} \times 5 \times 13 . Therefore, the following are factors of 2340 2340 less than or equal to 25 25 :

{ 1 , 2 , 3 , 4 , 5 , 6 , 9 , 10 , 12 , 13 , 15 , 18 , 20 } \left\{ 1,2,3,4,5,6,9,10,12,13,15,18,20 \right\}

There are 13 13 factors out of 25 25 total numbers. The probability that the second number is a factor of 2340 2340 is 13 25 \frac{13}{25} . Therefore, the probability of both outcomes occurring consecutively is:

1 4 × 13 25 = 13 100 = H 100 \frac{1}{4}\times \frac{13}{25}=\frac{13}{100}=\frac{H}{100}

We now know that H = 13 H = 13 . This gives us enough information to look at the problem about the 4-digit integer.

Since W X = Z Y \overline{WX}=\overline{ZY} , W X Y Z = W X X W \overline{WXYZ}=\overline{WXXW} .

Since W + X + X + W = 2 ( W + X ) W+X+X+W=2(W+X) is a power of 2 2 , W + X W+X must also be a power of two (a power of 2 2 divided by 2 2 is still a power of 2 2 ).

Since W × X × X × W = ( W × X ) 2 W \times X \times X \times W = (W\times X)^{2} is a power of 3 3 , W × X W\times X is also a power of 3 3 . It is easy to show that there are only three 4-digit integers for which W + X W+X is a power of 2 2 and W × X W\times X is a power of 3 3 :

{ 1111 , 1331 , 3113 } \left\{1111,1331,3113 \right\}

The problem states that W X X W \overline{WX}\neq \overline{XW} and that W X X W \overline{WXXW} has H D B = 2 H-D-B=2 distinct prime factors. This leaves 3113 3113 as the only possibility. Therefore, E = 31 E=31 .

We can now go to the polynomial.

f ( x ) = D H x 5 D 2 E H 2 x 4 + F G F x 3 C F D E F H x 2 + E H x D E 2 H 2 . f(x) = DHx^{5} - D^{2}EH^{2}x^{4} + FG^{F}x^{3} - C^{F}DEFHx^{2} + EHx - DE^{2}H^{2}.

Substituting in all known variables:

f ( x ) = 65 x 5 130975 x 4 + 324 x 3 652860 x 2 + 403 x 812045. f(x) = 65x^{5} - 130975x^{4} + 324x^{3} - 652860x^{2} + 403x - 812045.

This polynomial has only one real root N N : 2015 \boxed{2015}

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