<s>[INST] If $a$ is a root of $x^2+ x + 1$, simplify $1 + a + a^2 +\dots+ a^{2017}.$ 
If $a$ is a root of $x^2 + x + 1$ simplify $$1 + a + a^2 + a^3 + \cdots + a^{2017}.$$

my solution initially starts with the idea that $1 + a + a^{2} = 0$ since $a$ is one of the root then using the idea i grouped $$1 + a + a^2 + a^3 + \cdots + a^{2017}$$ just like this $$(1 + a + a^{2}) + a^3(1 + a + a^2) + a^6(1 + a + a^2) + \cdots + a^{2013} + (1 + a + a^2) + a^{2016} + a^{2017}$$
which is equivalent to $$(0) + a^3(0) + a^6(0) + \cdots + a^{2013}(0) + a^{2016} + a^{2017}$$ and finally simplified into $$a^{2016} + a^{2017}$$. That's my first solution, but eventually I noticed that since $1 + a + a^2 = 0$ then it can be $a + a^2 = -1 $ using this idea I further simplified $$a^{2016} + a^{2017}$$ into $$a^{2015}(a + a^{2})$$ ==> $$a^{2015}(-1)$$ ==> $$-a^{2015}$$ But answers vary if I start the grouping at the end of the expression resulting to $1 + a$ another solution yields to $-a^{2017}$ if I set $1 + a = -a^2$ several solutions rise when the grouping is being made anywhere at the body of the expression... my question is, Is $$a^{2016} + a^{2017} = 1 + a = -a^{2015} = -a^{2017} \text{ etc.?}$$ How does it happen? I already forgot how to manipulate complex solutions maybe that's why I'm boggled with this question...any help?
 [/INST] I haven't looked through your working, but here's a much easier method:
If $a^2+a+1=0$, then $(a-1)(a^2+a+1)=0$, that is, $a^3-1=0$, ie, $a^3=1$.
On the other hand, $1+a+\dots+a^{2017}=\frac{a^{2018}-1}{a-1}$. Since $a^3=1$, it follows that $a^{2018}=a^2$. So, your sum simplifies to $\frac{a^2}{a-1}$. <-- wrong! Read on....
Edit: 
I had a typo (thanks @Math lover for pointing that out). The sum simplifies to $\frac{a^2-1}{a-1}$, which simplifies easily to $a+1$.
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