Question 706842
{{{ 1+x+x^2+x^3+x^4+x^5+x^6+x^7 }}} is the sum of a geometric sequence.
I always rediscover the formula for a sum, because it's just as easy as looking it up a cumbersome formula and applying it.
{{{S=1+x+x^2+x^3+x^4+x^5+x^6+x^7}}}
{{{Sx=x+x^2+x^3+x^4+x^5+x^6+x^7+x^8}}}
{{{Sx-S=x^8-1}}} --> {{{S(x-1)=x^8-1}}} --> {{{S=highlight((x^8-1)/(x-1))}}}