The five real numbers $v,w,x,y,s$ satisfy the system of equations \begin{align*} v&=wx+ys,\\ v^2&=w^2x+y^2s,\\ v^3&=w^3x+y^3s. \end{align*}Show that at least two of them are equal.
Problem
Source: Germany 2024, Problem 1
Tags: system of equations, algebra, algebra proposed
NO_SQUARES
14.06.2024 22:37
Note that $(wx+ys)(w^3x+y^3s)=v^4 = (w^2x+y^2s)^2$, so $(wx)(ys)(y-w)^2=0$. If $y=w$, we have won. In addition, for reasons of symmetry, it is now sufficient to analyze only one case: $wx=0$ and $x,y,s \not = 0$. Then we have $v=ys \Rightarrow s=\frac{v}{y}$. Now $v^2=y^2s=y^2 \cdot \frac{v}{y}=vy \Rightarrow v(y-v)=0$ and so $y=v$.
onyqz
23.08.2024 15:03
Take $x$, $s$ as variables and $v,w,y$ as coefficients.
Consider the augmented coefficient matrix:
$$(A|b)=\left(\begin{array}{cc|c}
w & y & v\\
w^2 & y^2 & v^2\\
w^3 & y^3 & v^3\\
\end{array}\right) \sim
\left(\begin{array}{cc|c}
w & y & v \\
0 & y^2-wy & v^2-wv \\
0 & 0 & v^3-wv^2-v^2y+wvy\\
\end{array}\right)$$Now we use following Theorem:
Kronecker-Capelli's Theorem: Let $A$ be a coefficient matrix and $(A|b)$ be an augmented coefficient matrix w.r.t a system of equations. Then this system of equations has (at least) one solution if and only if $\text{rank}(A) = \text{rank}(A|b)$.
Since $v,w,x,y,s$ should satisfy the above system of equations, we must have $2=\text{rank}(A) = \text{rank}(A|b)$, so $v^3-wv^2-v^2y+wvy = v(v^2-(w+y)v+wy)=0$.
Case 1: $v=0$
Then $wx=-ys$ and thus $0=w^2x-wxy=wx(w-y)$. Either $w$ or $x$ is $0$ (which would give $v=w$ or $v=x$) or $w-y= 0$ (which would give $w=y$).
Case 2: $v^2-(w+y)v+wy=0$.
Solving this quadratic equation in $v$ gives either $v=w$ or $v=y$.
Therefore, there exist (at least) two of $v,w,x,y,s$ that are equal. $\square$