Complex numbers - Exponential numbers - Proof Let $z$ be a complex number, and let $n$ be a positive integer such that $z^n = (z + 1)^n = 1$. Prove that $n$ is divisible by 6.
For this problem I am stumped...how should I begin?
Also there's a hint for it:
From $z^n = 1$, prove that $|z| = 1$. What does the equation $(z + 1)^n = 1$ tell you? What do the resulting equations tell you about $z$?
Could someone give me a hint on where to begin? thanks in advance
 A: Hint 1: $|z-z_0|=r$ is a circle with radius $r$ and center $z_0$;
Hint 2: the unit circle is $z=e^{i\theta}$;
Hint 3: for any $n\in\Bbb{N}$,   $(e^{i\theta})^n=e^{in\theta}$;
Hint 4: if $\cos \theta= \pm 1$ and $\sin \theta =0$ then $\theta=k\pi$ for some integer $k$;
Hint 5: if $a,b,c$ are integers such that $a|bc$ and $\gcd(a,b)=1$ then $a|c$.
A: A problem way too cool and cute to pass up, so check this out:
$(1.) \; z^n = 1 \Rightarrow \vert z \vert^n = 1, \tag{1}$
$(2.) \; \vert z \vert^n = 1 \Rightarrow \vert z \vert = 1 \Rightarrow \exists \theta \in \Bbb R  \;\text{such that} \; z = e^{i\theta}, \tag{2}$
$(3.) \; \vert z \vert = 1 \Rightarrow z \bar z = 1, \tag{3}$
$(4.) \; (z + 1)^n = 1 \Rightarrow \vert z + 1 \vert^n = 1 \Rightarrow \vert z + 1\vert = 1, \tag{4}$
$(5.) \; \vert z + 1 \vert = 1 \Rightarrow (1 + z)(1 + \bar z) = 1, \tag{5}$
$(6.) \;  (1 + z)(1 + \bar z) = 1 \Rightarrow z \bar z + z + \bar z + 1 = 1 \Rightarrow z \bar z + z + \bar z = 0, \tag{6}$
$(7.) \; \text{by (3) and (6),} \; z + \bar z = -1, \tag{7}$
$(8.) \; \text{by (2) and (7),} \; 2 \Re{z} = 2 \cos \theta = -1 \Rightarrow \cos \theta = -\dfrac{1}{2}, \tag{8}$
$(9.) \; \cos \theta = -\dfrac{1}{2} \Rightarrow \Re{(1 + z)} = 1 + \cos \theta = \dfrac{1}{2}, \tag{9}$
$(10.) \; \Re{(1 + z)} = \dfrac{1}{2} \; \text{and} \; \vert 1 + z \vert = 1 \Rightarrow 1 + z = e^{(\pm 2\pi i/ 6) + 2k \pi}, k \in \Bbb Z, \tag{10}$
$(11.) \;  1 + z = e^{(\pm 2\pi i/ 6) + 2k \pi} \Rightarrow 1 = (1 + z)^n = e^{\pm 2n \pi i / 6}, \tag{11}$
$(12.) \; e^{\pm 2n \pi i / 6} = 1 \Rightarrow 6 \mid n. \tag{12}$
QED
A: We assume $r,m,v$ to be integers
Using de Moivre's theorem,
$$z^n=1=e^{2r\pi i}\implies z=e^{\dfrac{2r\pi i}n}=\cos\frac{2r\pi}n+i\sin \dfrac{2r\pi}n$$
From this, $$z^n=(1+z)^n\implies z=-\frac12-i\frac{\cot\dfrac{m\pi}n}2$$
So, equating the real parts, $\displaystyle\cos\frac{2r\pi}n=-\frac12$
$\displaystyle\implies\sin\dfrac{2r\pi}n=\pm\sqrt{1-\left(-\frac12\right)^2}=\pm\frac{\sqrt3}2$ 
Equating the imaginary part,
$$-\frac{\cot\dfrac{m\pi}n}2=\pm\frac{\sqrt3}2\implies\tan\dfrac{m\pi}n=\mp\frac1{\sqrt3}=\tan\left(\mp\frac\pi6\right)$$
$$\implies \dfrac{m\pi}n=v\pi\mp\frac\pi6\iff 6m=n(6v\mp1)$$
We have $\displaystyle\frac{n(6v\mp1)}6=m$ which is an integer
But as $\displaystyle(6v\mp1,6)=1, 6$ must divide $n$
A: We assume $u,v,m,p $ to be integers
Using de Moivre's theorem and Euler Formula,
$$z^n=1=e^{2u\pi i}\implies z=e^{\dfrac{2u\pi i}n}=\cos\frac{2u\pi}n+i\sin \dfrac{2u\pi}n$$
$$\text{and }(z+1)^n=1\implies z+1=\cos\frac{2v\pi}n+i\sin\dfrac{2v\pi}n$$
$$\implies \cos\frac{2u\pi}n+i\sin\dfrac{2u\pi}n=\cos\frac{2v\pi}n-1+i\sin\dfrac{2v\pi}n$$
Equating the real parts, $\displaystyle\cos\dfrac{2u\pi}n=\cos\dfrac{2v\pi}n-1\ \ \ \  (1)$
Again,  equating the imaginary parts, $\displaystyle\sin\dfrac{2u\pi}n=\sin\dfrac{2v\pi}n \ \ \ \  (2)$
$\displaystyle\implies\cos\dfrac{2u\pi}n=\pm\cos\dfrac{2v\pi}n$
Taking the '+' sign,  $(1)$ reduces to $\displaystyle\cos\dfrac{2v\pi}n=\cos\dfrac{2v\pi}n-1\iff 0=-1$
Taking the '-' sign, $(1)$ reduces to $\displaystyle-\cos\dfrac{2v\pi}n=\cos\dfrac{2v\pi}n-1\iff\cos\dfrac{2v\pi}n=\frac12=\cos\frac\pi3$
$\displaystyle\implies\dfrac{2v\pi}n=2p\pi\pm \frac\pi3\implies \frac{n(6p\pm1)}6=v$ which is an integer (Now, the last logic is omitted as is available  from my other answer)
One observation:
$z$ is actually $\omega$ where $\omega$ is a Complex Cube root of Unity
Also try squaring & adding $(1),(2)$
A: We can say: $$z^n=1 \implies |z|^n=1 \implies |z^n|=1 \implies |z|=1$$
Using similar logic, we can also get: $$(z+1)^n=1 \implies |z+1|^n=1 \implies |(z+1)^n|=1 \implies |z+1|=1$$
Let's set $z=a+bi$: $$|z|=|z+1| \implies a^2 + b^2 = (a+1)^2 + b^2 \implies a^2 + b^2 = a^2 + b^2 + 2a + 1$$ $$\implies 2a + 1 = 0 \implies a = -\frac{1}{2}$$
Pluging this back into the equation, we get: $$\left(-\frac{1}{2}\right)^2 + b^2 = 1 \implies b^2 = \frac{3}{4} \implies b = \pm \frac{\sqrt{3}}{2}$$
With this information, we know that: $$z= -\frac{1}{2} \pm \frac{\sqrt{3}}{2}i \implies z = e^{\pm 2\pi i/3}$$
Also: $$z+1= \frac{1}{2} \pm \frac{\sqrt{3}}{2}i \implies z+1 = e^{\pm \pi i/3} \implies (z+1)^n = e^{\pm \pi ni/3}$$
For $(z+1)^n$ to be $1$, $\cos \pm \pi n/3 = 1$ and $\sin \pm \pi n/3 = 0$. This can only happen if $\pm \pi n/3 = 2\pi k$ for some integer $k$
Solving for $n$, we get: $$\pm \pi n/3 = 2\pi k \implies n = \pm 6k \implies \boxed{6 \mid n}.$$
