System of equations

Algebra Level 5

{ x 2 y + y 2 z + z 2 x = 2186 x y 2 + y z 2 + z x 2 = 2188 \large \begin{cases} x^2y+ y^2z+ z^2x = 2186 \\ xy^2+ yz^2 + zx^2 = 2188 \end{cases}

Suppose x x , y y , and z z are integers that satisfy the system of equations above. Find x 2 + y 2 + z 2 = ? x^2+y^2+z^2= ?


The answer is 245.

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1 solution

Kelvin Hong
Aug 11, 2017

I saw that

x 2 y + y 2 z + z 2 x = 2186 = 2187 1 = 3 7 1 x^2y+y^2z+z^2x=2186=2187-1=3^7-1

x y 2 + y z 2 + z x 2 = 2188 = 2187 + 1 = 3 7 + 1 xy^2+yz^2+zx^2=2188=2187+1=3^7+1

so I first to solve

x 2 y + y 2 z + z 2 x = x y 2 + y z 2 + z x 2 = 3 7 x^2y+y^2z+z^2x=xy^2+yz^2+zx^2=3^7

I get ( 9 , 9 , 9 ) (9,9,9)

Besides that, I found that , for the original question, it is obviously can't have x = y = z x=y=z because 2186 2188 2186\neq2188

if I let x = y x=y , then x 3 + x 2 z + x z 2 = 2186 x^3+x^2z+xz^2=2186 and x 3 + x z 2 + x 2 z = 2188 x^3+xz^2+x^2z=2188 also leads to contradiction.

Same as when y = z y=z and z = x z=x .

So for the original question, x y z x\neq y \neq z .

I test 8 , 9 , 10 8,9,10 try to get 3 7 1 3^7-1 and 3 7 + 1 3^7+1 by both equation, then it holds true!

x 2 + y 2 + z 2 = 64 + 81 + 100 = 245 x^2+y^2+z^2=64+81+100=\boxed{245}

Thank you for sharing a nice and logical solution.

Hana Wehbi - 3 years, 10 months ago

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Ya ,you are welcome

Kelvin Hong - 3 years, 10 months ago

Nice solution. It was like a false position method. I'm guessing you got (9, 9, 9) by guessing x = y = z? It says 59% of people got this right. I wonder how they did it. And how could we know the solution is unique?

James Wilson - 3 years, 9 months ago

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No, I guess (9,9,9) is for approximately solution, but not accurate solution. I guess (9,9,9) as a starting point to find actual answer. I think that is a method to find out (8,9,10) is the unique answer.

Kelvin Hong - 3 years, 9 months ago

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Yes, I know that. You misunderstood me. Look up "method of false position." I was just guessing at how you came up with (9, 9, 9), but that doesn't really matter.

James Wilson - 3 years, 9 months ago

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@James Wilson Oh, how I came up with (9,9,9)... I initially don't know how to start to solve this problem, so I let it three be the same, then it is what you see ... This solution isn't rigorous, also.

Kelvin Hong - 3 years, 9 months ago

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@Kelvin Hong Okay. 13 people solved it so far. I wonder if they did it the same way.

James Wilson - 3 years, 9 months ago

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@James Wilson Haha, I hope one of them has full solution! But what about your solution?

Kelvin Hong - 3 years, 9 months ago

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@Kelvin Hong I decided to just view the answer.

James Wilson - 3 years, 9 months ago

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@James Wilson I wish I had been able to solve it. I'm going to go cry now. (just kidding)

James Wilson - 3 years, 9 months ago

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@James Wilson Owhh, haha.

Kelvin Hong - 3 years, 9 months ago

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