<s>[INST] Prove that $\Delta ABC$ is an equilateral triangle if and only if the complex coordinates $a$,$b$, and $c$ satisfy a relation. 
Let $\triangle ABC$ be a triangle in the complex plane and let $a$,$b$ and $c$, respectively, be the complex coordinates of its vertices.  Suppose that the tiangle is inscribed in the circle $C(0,1)$.  Prove that $\triangle ABC$ is equilateral if and only if $$\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}=0$$ and $$(a+b)(b+c)(c+a)\neq0\,.$$

I tried to convert $\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}=0$ into a simpler equation by multipling it by $(a+b)(b+c)(c+a)$
In the end I obtained $$a^2+b^2+c^2+3ab+3bc+3ca=0$$ but I don't know how to continue from here.  Can you help me, please?
 [/INST] Given equilateral triangle, i.e.
$$a= e^{i\alpha}, \>\>\>\>\>b = e^{i(\alpha+\frac{2\pi} 3)}, \>\>\>\>\>c= e^{i(\alpha-\frac{2\pi} 3)}
$$
it is straightforward to verify $a^2+b^2+c^2+3ab+3bc+3ca=0$.
Conversely, given
$$\frac{1}{a+b}+\frac{1}{b+c}+\frac{1}{c+a}=0$$
rewrite the equation in term of $b =a e^{i x}$,  $c =a e^{ i y} $
$$\frac{1}{1+ e^{i x}}+\frac{1}{1+ e^{i y}}+\frac{1}{e^{i x} + e^{i y}}=0\tag1$$
Suppose $(x,y)$ is a solution, then $(y,x)$ is also a solution due to party, as well as $(-x,-y)$ via conjugate. which implies $x = -y$. Substitute into (1) to obtain $\cos x=-\frac12$, or $x= -y= \pm \frac {2\pi}3$. Thus
$$b = a e^{\pm i\frac{2\pi}3}, \>\>\>\>\>c=a  e^{\mp i \frac{2\pi} 3}
$$
hence, equilateral triangles.
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