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Geometry Level 4

Let x x , y y and z z be the sides of a traingle. Then let the perimeter of the triangle be 466 466 , let the area of the triangle divided by the square root of the perimeter be 12834 \sqrt{12834} , let the magnitude of side x x and let the semi perimeter minus the magnitude of side x x be the smallest root of the polynomial x 3 9 x 2 + 26 x 24 x^3 - 9x^2 + 26x - 24 . Then, if the magnitude of side x x can be expressed as ( a ) b c (-a)bc , the magnitude of side y y can be expressed as 10 ( a + b + c ) 10(a + b + c) and the magnitude of side z z can be expressed as 20 ( a b + a c + b c ) 20(ab + ac + bc) , find a , b a,b and c c . Submit your answer as the sum of the digit sums of a , b a,b and c c , where the digit sum includes the digits both sides of the decimal point (for example, the digit sum of 12.5 -12.5 is 8 8 ).


The answer is 21.

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1 solution

Guilherme Niedu
Sep 1, 2019

By the Rational Root Theorem , if the roots of the polynomial x 3 9 x 2 + 26 x 24 x^3 - 9x^2 + 26x - 24 are rational, they'll be in the set ( ± 1 , ± 2 , ± 3 , ± 4 , ± 6 , ± 8 , ± 12 , ± 24 ) (\pm1, \pm2, \pm3, \pm4, \pm6, \pm8, \pm12, \pm24 ) . Checking it manually, it's possible to see that 2 2 , 3 3 and 4 4 are roots.

So, the smallest root is 2 2 and the condition of the semi-perimeter gives:

466 2 x = 2 \large \displaystyle \frac{466}{2} - x = 2

x = 231 \color{#20A900} \boxed { \large \displaystyle x = 231 }

The equation of the perimeter is, then:

x + y + z = 466 \large \displaystyle x + y + z = 466

y + z = 235 \large \displaystyle y + z = 235

The condition of the ratio between the area and the square root of the perimeter, using the triangle's area general formula gives:

1 4 ( x + y + z ) ( x + y z ) ( x y + z ) ( x + y + z ) ( x + y + z ) = 12834 \large \displaystyle \frac{ \frac14 \sqrt{ \left ( x+y+z \right ) \cdot \left ( x+y-z \right ) \cdot \left ( x-y+z \right ) \cdot \left ( -x+y+z \right ) } } {\sqrt{ \left ( x+y+z \right )}} = \sqrt{12834}

The perimeter inside the numerator will cancel out with the denominator:

( x + y z ) ( x y + z ) ( x + y + z ) = 4 12834 \large \displaystyle \sqrt{ \left ( x+y-z \right ) \cdot \left ( x-y+z \right ) \cdot \left ( -x+y+z \right ) } = 4 \sqrt{12834}

The terms inside the square root can be rewritten as:

( x + y + z 2 z ) ( x + y + z 2 y ) ( x + y + z 2 x ) = 4 12834 \large \displaystyle \sqrt{ \left ( x+y+z - 2z \right ) \cdot \left ( x+y+z - 2y \right ) \cdot \left ( x+y+z - 2x \right ) } = 4 \sqrt{12834}

( 466 2 z ) ( 466 2 y ) ( 4 ) = 4 12834 \large \displaystyle \sqrt{ \left ( 466 - 2z \right ) \cdot \left ( 466 - 2y \right ) \cdot ( 4 ) } = 4 \sqrt{12834}

( 466 2 z ) ( 466 2 y ) 4 = 16 12834 \large \displaystyle \left ( 466 - 2z \right ) \cdot \left ( 466 - 2y \right ) \cdot 4 = 16 \cdot 12834

217156 466 2 ( y + z ) + 4 y z = 51336 \large \displaystyle 217156 - 466 \cdot 2 \cdot (y+z) + 4yz = 51336

Since y + z = 235 y + z = 235 :

y z = 13300 \large \displaystyle yz = 13300

Solving:

y z = 13300 \large \displaystyle yz = 13300

y + z = 235 \large \displaystyle y + z = 235

One gets:

y = 140 , z = 95 \color{#20A900} \boxed{ \large \displaystyle y = 140}, \boxed{ \large \displaystyle z = 95}

Or:

y = 95 , z = 140 \color{#20A900} \boxed{ \large \displaystyle y = 95}, \boxed{ \large \displaystyle z = 140}

Solving for the first condition ( y = 140 y= 140 , z = 95 z = 95 ):

a b c = 231 \large \displaystyle -abc= 231

10 ( a + b + c ) = 140 \large \displaystyle 10(a+b+c) = 140

20 ( a b + a c + b c ) = 95 \large \displaystyle 20(ab+ac+bc) = 95

One gets six possible solutions:

a = 3.5 , b = 5.5 , c = 12 \color{#20A900} \boxed{ \large \displaystyle a = -3.5, b = 5.5, c = 12}

a = 3.5 , b = 12 , c = 5.5 \color{#20A900} \boxed{ \large \displaystyle a = -3.5, b = 12, c = 5.5}

a = 5.5 , b = 3.5 , c = 12 \color{#20A900} \boxed{ \large \displaystyle a = 5.5, b = -3.5, c = 12}

a = 5.5 , b = 12 , c = 3.5 \color{#20A900} \boxed{ \large \displaystyle a = 5.5, b = 12, c = -3.5}

a = 12 , b = 3.5 , c = 5.5 \color{#20A900} \boxed{ \large \displaystyle a = 12, b = -3.5, c = 5.5}

a = 12 , b = 5.5 , c = 3.5 \color{#20A900} \boxed{ \large \displaystyle a = 12, b = 5.5, c = -3.5}

Solving for the second condition ( y = 95 y= 95 , z = 140 z = 140 ), a a , b b and c c will have non-real solutions.

So, the only possible solutions for a a , b b and c c are the ones with:

x = 231 , y = 140 , z = 95 \color{#3D99F6} \boxed{ \large \displaystyle x=231, y=140, z = 95}

Also, ( a , b , c ) (a,b,c) will be permutations of the set ( 12 , 5.5 , 3.5 ) (12, 5.5, -3.5 )

And the digit sum of a a , b b and c c in any of the six cases, i.e., in any of the permutations is:

3 + 5 + 5 + 5 + 1 + 2 = 21 \color{#3D99F6} \boxed{ 3+5+5+5+1+2 = 21}

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