Find all real solutions

Algebra Level 3

Find the number of real solutions ( x , y , z ) (x,y,z) of

{ ( x + y ) 3 = z ( y + z ) 3 = x ( z + x ) 3 = y \begin{cases} (x+y)^{3}=z \\ (y+z)^{3}=x \\ (z+x)^{3}=y \end{cases}


The answer is 3.

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4 solutions

Ravi Teja
Sep 30, 2014

Without loss of generality, lets assume; x>=y>=z. adding x on all 3 sides: 2x>=x+y>=x+z adding y on all 3 sides: x+y>=2y>=y+z. Adding z on all three sides: x+z>=y+z>=2z.

Cubing on all three sides, and substituting for their respective values from the problem statement we get; 8x^3>=z>=y z>=8y^3>=x y>=x>=8z^3

we get z>=y>=x and since we assumed x>=y>=z; the only plausible soln is x=y=z. Substitiuting; we get three solutions..

Ramesh Gupta
Sep 30, 2014

This is a cyclic equation. So for solving lets put x=y=z we got (x+x)^3=x means 8x^3 -x=0

Which has three solution

nyc solution....

Pradyumna Kabra - 6 years, 8 months ago
Catalin Popescu
Jul 11, 2020

If we subtract second equation from first we get (x-z)(a^2 +ab+b^2) = z-x, where a = x+y and b = y+z. So, we either have x = z, or a^2 + ab + b^2 = -1, but the latter is not possible, as a^2 + ab + b^2 = (a+b/2)^2 + 3(b^2)/4 > 0. Hence x = z and similarly y = z = x.

Jaiveer Shekhawat
Sep 29, 2014

TRULY SPEAKING IT WAS MY LUCK WHICH HELPED ME TO GET THE ANSWER CORRECT IN THE SECOND TRIAL.... SO IF ANYONE OF YOU KNOW THE METHOD PLEASE SHARE WITH ALL OF US!!!

calm down...

Patrick Engelmann - 6 years, 8 months ago

My was the first trial 😆

Mayank Mangla - 4 years, 3 months ago

My third lol!

Tấn Phát Nguyễn - 3 years, 7 months ago

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