I am struggling to understand a step in the accepted solution of 
[How Can One Prove $\cos(\pi/7) + \cos(3 \pi/7) + \cos(5 \pi/7) = 1/2$](https://math.stackexchange.com/questions/140388/how-can-one-prove-cos-pi-7-cos3-pi-7-cos5-pi-7-1-2).

I have been new to this site, I can not figure out where to ask this question.

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> Can someone please enlighten me why $u^5+u^3+u=a$ is equivalent to $ua+1=a$? 
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With algebra, I just get stuck. I also can't find any property that establishes the equivalence.

Thanks in advance.