Y = x+x^2+x^3+x^4.....x^n ----------------(1)
xY = x^2+x^3+x^4.....x^n+x^(n+1)--------(2)
(2)-(1) you get:
(x-1)Y = x^(n+1)-x
so: Y =(x^(n+1)-x)/(x-1)