⎩ ⎨ ⎧ x 2 y + y 2 z + z 2 x = 2 1 8 6 x y 2 + y z 2 + z x 2 = 2 1 8 8
Suppose x , y , and z are integers that satisfy the system of equations above. Find x 2 + y 2 + z 2 = ?
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Thank you for sharing a nice and logical solution.
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?
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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.
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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.
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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.
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@Kelvin Hong – Okay. 13 people solved it so far. I wonder if they did it the same way.
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@James Wilson – Haha, I hope one of them has full solution! But what about your solution?
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@Kelvin Hong – I decided to just view the answer.
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@James Wilson – I wish I had been able to solve it. I'm going to go cry now. (just kidding)
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I saw that
x 2 y + y 2 z + z 2 x = 2 1 8 6 = 2 1 8 7 − 1 = 3 7 − 1
x y 2 + y z 2 + z x 2 = 2 1 8 8 = 2 1 8 7 + 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
I get ( 9 , 9 , 9 )
Besides that, I found that , for the original question, it is obviously can't have x = y = z because 2 1 8 6 = 2 1 8 8
if I let x = y , then x 3 + x 2 z + x z 2 = 2 1 8 6 and x 3 + x z 2 + x 2 z = 2 1 8 8 also leads to contradiction.
Same as when y = z and z = x .
So for the original question, x = y = z .
I test 8 , 9 , 1 0 try to get 3 7 − 1 and 3 7 + 1 by both equation, then it holds true!
x 2 + y 2 + z 2 = 6 4 + 8 1 + 1 0 0 = 2 4 5