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Algebra Level 5

If { x y = 61741 y z = 14197 x 1 / 7 + y 1 / 7 + z 1 / 7 = 12 \begin{cases} x-y = 61741 \\ y-z = 14197 \\ {x}^{1/7}+{y}^{1/7}+{z}^{1/7} =12 \end{cases}

Calculate x + y + z x+y+z


The answer is 96696.

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3 solutions

Chew-Seong Cheong
Dec 30, 2014

From x 1 7 + y 1 7 + z 1 7 = 12 x^{\frac{1}{7}} + y^{\frac{1}{7}} + z^{\frac{1}{7}} = 12 , we know that x x , y y and z z must be integers of perfect 7 7 -power.

We also know that y = x 61741 y = x - 61741 , z = x 75938 z = x -75938 and z < y < x < 1 2 7 z < y < x < 12^7 . We can list out n 7 n^7 for n = 1 , 2 , 3...12 n = 1,2,3...12 and find for n such ( n 7 61741 ) (n^7 - 61741) and ( n 7 75938 ) (n^7 -75938) are also perfect 7 7 -powers. It is readily done with a spreadsheet and mine is as follows:

n n 7 n 7 61741 n 7 75938 1 1 2 128 3 2187 4 16384 5 78125 16384 2187 6 279936 218195 203998 7 823543 761802 747605 8 2097152 2035411 2021214 9 4782969 4721228 4707031 10 10000000 9938259 9924062 11 19487171 19425430 19411233 12 35831808 35770067 35755870 \begin {matrix} n & n^7 & n^7 -61741 & n^7-75938 \\ 1 & 1 & & \\ 2 & 128 & & \\ 3 & \boxed{2187} & & \\ 4 & \boxed{16384} & & \\ 5 &78125 & \boxed {16384} & \boxed {2187} \\ 6 & 279936 & 218195 & 203998 \\ 7 & 823543 & 761802 & 747605 \\ 8 & 2097152 & 2035411 & 2021214 \\ 9 & 4782969 & 4721228 & 4707031 \\ 10 & 10000000 & 9938259 & 9924062 \\ 11 &19487171 & 19425430 & 19411233 \\ 12 & 35831808 & 35770067 & 35755870 \end {matrix}

It can be seen that x = 5 7 x = 5^7 , y = 4 7 y = 4^7 and z = 3 7 z = 3^7 . Therefore,

x + y + x = 5 7 + 4 7 + 3 7 = 78125 + 16384 + 2187 = 96696 x+y+x = 5^7 + 4^7 + 3^7 = 78125+16384+2187 = \boxed{96696 }

The question did not state that x , y , z x,\text{ }y, \text{ }z were integers. How would you prove it? Or at least proof that there is only 1 1 solution to x + y + z x+y+z ?

Julian Poon - 6 years, 5 months ago

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I believe we can plot a graph to show it. Though may not proving it. I am attaching the plot here. It is for 13000 x 13000 -13000 \le x \le 13000 . Clearly there is only one solution for x x .

Chew-Seong Cheong - 6 years, 5 months ago

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wow. thats pretty interesting.

Julian Poon - 6 years, 5 months ago

Suppose its real , then x or y or z can be written as a fraction - such that both the numerator and the denominator are prime numbers , now 7th root will give us an irrational number , but on RHS we are having an integer , thus x,y,z varies over whole numbers

U Z - 6 years, 5 months ago
Kanireddy Sandeep
Dec 23, 2014

When we distribute 12 as sum of three integers, then from all the possibilities we can remove those sets of integers which have : 3 or 2 same integers. 1 as an integer. All even integers. Thus we get 3+4+5. As per conditions given in question , when crosschecked we will get x^1/7=5, y^1/7=4, z^1/7=3.

And we all used bashing....

Julian Poon - 6 years, 5 months ago

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this striked but then gave off

U Z - 6 years, 5 months ago

Where do these conditions come from?

Sarthak Tanwani - 6 years, 5 months ago
Incredible Mind
Jan 2, 2015

look the answer upside down

@incredible mind Not true!!!! Looking upside down, the answer comes out to be 96996

Aaghaz Mahajan - 2 years, 12 months ago

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