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[itex]1-x+x^2-x^3+..[/itex]

i don't understand why i can't do this?

i have [itex]\sum_{k=1}^{n} ar^k=\frac{a(r^{n+1}-r)}{r-1}[/itex]

ok but then when i sub in i get messed up i get [itex]\frac{(-x)^{n+1}+x}{-(1+x)}[/itex] is there any way of simplifying this? also when i set n=1 i get that the sum is -x when it should be 1-x???