New Year's Countdown Day 4: Fourth Roots

Algebra Level 3

If

1009 + 2017 4 + 1009 2017 4 = a + b c 4 , \sqrt[4]{1009 + \sqrt{2017}} + \sqrt[4]{1009 - \sqrt{2017}} = \sqrt[4]{a + b \sqrt{c}},

where a , b , c a, b, c are positive integers such that c c is not divisible by the square of any prime, then find the value of c a . c - a.


This problem is part of the set New Year's Countdown 2017 .


The answer is 20172.

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2 solutions

Let, m = 1009 + 2017 4 , n = 1009 2017 4 We need to find m + n m 4 + n 4 = 2018 ( m n ) 2 = 1008 ( m 2 + n 2 ) 2 = ( m 4 + n 4 + 2 ( m n ) 2 ) = 4034 ( m 2 + n 2 ) = 4034 ( 1 ) m n = 1008 ( 2 ) ( m + n ) 2 = ( m 2 + n 2 + 2 m n ) = ( 4034 + 4032 ) From ( 1 ) and ( 2 ) ( m + n ) = ( 4034 + 4032 ) Let, u = 4034 v = 4032 ( u + v ) 2 = 8066 + 2 4034 × 4032 = 8066 + 48 28238 ( m + n ) = ( u + v ) = 8066 + 48 28238 4 On comparing, we get a = 8066 , b = 48 , c = 28238 c a = 28238 8066 = 20172 \begin{aligned}\text{Let,}\\ m&= \sqrt[4]{1009 +\sqrt{2017}},\\ n&=\sqrt[4]{1009 -\sqrt{2017}}\\ \text{We} &\text{ need to find } m+n\\\\ m^4+n^4&=2018\\ (mn)^2&=1008\\ (m^2+n^2)^2&=(m^4+n^4+2(mn)^2)=4034\\ \implies(m^2+n^2)&=\sqrt{4034}\hspace{4mm} \color{#3D99F6}(1)\\ mn&=\sqrt{1008}\hspace{4mm} \color{#3D99F6}(2)\\ (m+n)^2&=(m^2+n^2+2mn)=\left(\sqrt{4034}+\sqrt{4032}\right)\hspace{4mm}\color{#3D99F6}\text{From } (1) \text{ and } (2)\\ (m+n)&=\sqrt{\left(\sqrt{4034}+\sqrt{4032}\right)}\\\\ \text{Let,} u&=\sqrt{4034}\\ v&=\sqrt{4032}\\ (u+v)^2&=8066+2\sqrt{4034\times4032}\\ &=8066+48\sqrt{28238}\\ (m+n)&=\sqrt{(u+v)}\\ &=\sqrt[4]{8066+48\sqrt{28238}}\\ \text{On }&\text{comparing, we get } a=8066,b=48,c=28238\\ \implies c-a&=28238-8066=\color{#EC7300}\boxed{\color{#333333}20172}\end{aligned}

x = 1009 + 2017 4 + 1009 2017 4 Let k = 2017 = k 2 + 1 2 + k 4 + k 2 + 1 2 k 4 = k 2 + 2 k + 1 2 4 + k 2 2 k + 1 2 4 = ( k + 1 ) 2 2 4 + ( k 1 ) 2 2 4 = k + 1 2 4 + k 1 2 4 = ( k + 1 2 4 + k 1 2 4 ) 2 = k + 1 + 2 k 2 1 + k 1 2 = 2 k + 2 k 2 1 2 = 2 4 k + k 2 1 = 2 4 ( k + k 2 1 ) 2 4 = 2 4 k 2 + 2 k k 2 1 + k 2 1 4 = 4 k 2 2 + 4 k k 2 1 4 Putting back k = 2017 = 4 ( 2017 ) 2 + 4 2017 ( 2016 ) 4 = 8066 + 48 28238 4 \begin{aligned} x & = \sqrt[4]{1009+\sqrt{2017}} + \sqrt[4]{1009-\sqrt{2017}} & \small \color{#3D99F6} \text{Let }k = \sqrt{2017} \\ & = \sqrt[4]{\frac {k^2+1}2+k} + \sqrt[4]{\frac {k^2+1}2-k} \\ & = \sqrt[4]{\frac {k^2+2k+1}2} + \sqrt[4]{\frac {k^2-2k+1}2} \\ & = \sqrt[4]{\frac {(k+1)^2}2} + \sqrt[4]{\frac {(k-1)^2}2} \\ & = \frac {\sqrt{k+1}}{\sqrt[4]2} + \frac {\sqrt{k-1}}{\sqrt[4]2} \\ & = \sqrt{\left(\frac {\sqrt{k+1}}{\sqrt[4]2} + \frac {\sqrt{k-1}}{\sqrt[4]2}\right)^2} \\ & = \sqrt{\frac {k+1+2\sqrt{k^2-1}+k-1}{\sqrt 2}} \\ & = \sqrt{\frac {2k+2\sqrt{k^2-1}}{\sqrt 2}} \\ & = \sqrt[4]2 \sqrt{k+\sqrt{k^2-1}} \\ & = \sqrt[4]2 \sqrt[4]{(k+\sqrt{k^2-1})^2} \\ & = \sqrt[4]2 \sqrt[4]{k^2+2k\sqrt{k^2-1}+k^2-1} \\ & = \sqrt[4]{4k^2-2+4k\sqrt{k^2-1}} & \small \color{#3D99F6} \text{Putting back }k = \sqrt{2017} \\ & = \sqrt[4]{4(2017)-2+4\sqrt{2017(2016)}} \\ & = \sqrt[4]{8066+48\sqrt{28238}} \end{aligned}

c a = 28238 8066 = 20172 \implies c - a = 28238 - 8066 = \boxed{20172}

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