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Let a > b > c a > b > c \; be the roots of the equation ( x 1 ) 3 + ( x + 3 ) 3 = 42 ( x + 1 ) \begin{aligned} (x-1)^3 + (x+3)^3 = 42(x+1) \end{aligned}

The value of

( a + b a a + b b ) ( a + b c + c ) ( 1 a + b + 1 a + c + 1 b + c ) ( a b + c ) \begin{aligned} \dfrac{\left (\dfrac{a+b}{a^a+b^b} \right )^{(a+\sqrt{bc}+c)}}{\left (\dfrac{1}{a+b}+\dfrac{1}{a+c}+\dfrac{1}{b+c} \right )^{(a-b+c)}} \end{aligned}

can be written as m n \dfrac{m}{n} , where m m and n n are positive coprime numbers. Evaluate m m + n m m 3 m m n 2 + 20 . \begin{aligned} \dfrac{\sqrt{m^m+n^m-m}}{3m^m-n^2+20}.\end{aligned}


The answer is 32.

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1 solution

Will McGlaughlin
Apr 26, 2018

First we want to find the roots of the equation, a, b, c.

To do this we will expand the equation and then factorise.

The equation expands to 2 x 3 + 6 x 2 12 x 16 2{ x }^{ 3 }+6{ x }^{ 2 }-12x-16

Factorising this we get the equation 2 ( x 2 ) ( x + 1 ) ( x + 4 ) 2(x-2)(x+1)(x+4)

The roots of this equation are x = 2 x=2 , x = 1 x=-1 , x = 4 x=-4

Therefore a = 2 a=2 , b = 1 b=-1 , c = 4 c=-4

Substituting into the equation

( a + b a a + b b ) ( a + b c + c ) ( 1 a + b + 1 a + c + 1 b + c ) ( a b + c ) \begin{aligned} \dfrac{\left (\dfrac{a+b}{a^a+b^b} \right )^{(a+\sqrt{bc}+c)}}{\left (\dfrac{1}{a+b}+\dfrac{1}{a+c}+\dfrac{1}{b+c} \right )^{(a-b+c)}} \end{aligned}

( 2 1 2 2 + 1 1 ) ( 2 + 1 ( 4 ) + c ) ( 1 2 1 + 1 2 4 + 1 1 4 ) ( 2 + 1 4 ) \begin{aligned} \dfrac{\left (\dfrac{2-1}{2^2+-1^{-1}} \right )^{(2+\sqrt{-1(-4)}+c)}}{\left (\dfrac{1}{2-1}+\dfrac{1}{2-4}+\dfrac{1}{-1-4} \right )^{(2+1-4)}} \end{aligned}

Which then simplifies to 1 10 3 \frac { 1 }{ \frac { 10 }{ 3 } }

10 3 = m n \frac { 10 }{ 3 } =\frac { m }{ n }

Substituting into the equation

m m + n m m 3 m m n 2 + 20 . \begin{aligned} \dfrac{\sqrt{m^m+n^m-m}}{3m^m-n^2+20}.\end{aligned}

1 0 10 + 3 10 10 3 ( 1 0 10 ) 3 2 + 20 . \begin{aligned} \dfrac{\sqrt{10^{10}+3^{10}-10}}{3(10^{10})-3^{2}+20}.\end{aligned}

Which simplifies to 1024 \sqrt { 1024 }

2 10 \sqrt { 2^{10} }

2 5 { 2 }^{ 5 }

= 32 \boxed{32}

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