# How do we solve the system of equations?18-1-2013

Giải hệ phương trình: (Editor's note: This translates to "Solve a system of equations:") $$\begin{cases}\sqrt{x}\left(\dfrac{1}{4}+\dfrac{2\sqrt{x}+\sqrt{y}}{x+y}\right)=2 \\[8pt] \sqrt{y}\left(\dfrac{1}{4}-\dfrac{2\sqrt{x}+\sqrt{y}}{x+y}\right) =1\end{cases}$$

• differential equations? – user52188 Jan 18 '13 at 0:48
• I added the algebra-precalculus tag, but didn't remove the DEs tag, just in case this somehow relates the DEs. I don't think it does, though. – apnorton Jan 18 '13 at 0:59
• "Pre-calculus" doesn't seem like an appropriate description. – Michael Hardy Jan 18 '13 at 1:01

Divide the first equation by $\sqrt{x}$ and the second by $\sqrt{y}$. Then adding and subtracting the two resulting equations gives us the new pair of simultaneous equations: $$\frac{2}{\sqrt{x}}+\frac{1}{\sqrt{y}}=\frac{1}{2} \\ \frac{2}{\sqrt{x}}-\frac{1}{\sqrt{y}}=2\frac{2\sqrt{x}+\sqrt{y}}{x+y} \, .$$
Multiplying these two equations together, we have: $$\frac{4}{\sqrt{x}}-\frac{1}{\sqrt{y}}=\frac{2\sqrt{x}+\sqrt{y}}{x+y} \, .$$ Clearing denominators yields: $$(4\sqrt{y}-\sqrt{x})(x+y)=2x\sqrt{y}+y\sqrt{x}$$ which after some algebra can be reduced to $$(x+2y)(2\sqrt{y}-\sqrt{x})=0 \, .$$
So either $x=-2y$ or $\sqrt{x}=2\sqrt{y}$. If $x$ and $y$ are positive and real the first is clearly impossible and the second is equivalent to $x=4y$. (If they're not positive and real, we have to worry more about branch cuts than I really want to.) Now, if $x=4y$ we have $$\frac{\sqrt{2}}{\sqrt{y}}+\frac{1}{\sqrt{y}}=\frac{1}{2} \, ,$$ from the top equation in this answer. So $\sqrt{y}=2(1+\sqrt{2})$, yielding the solution $$x=64(1+\sqrt{2})^4 \\ y=16(1+\sqrt{2})^4 \, ,$$ which upon expanding is precisely what Robert Israel got from Maple in the other answer.
According to Maple, there is one real solution, $$x = 1088+768 \sqrt{2},\ y = 272+192 \sqrt{2}$$