SERIE GEOMÉTRICA

Clear["Global`*"]

f[x_] := (1/(1 - x))

n := 3

s[x_] := (∑_(n = 0)^n) (x^n)

Plot[{f[x], s[x]}, {x, -2, 2}, 
      PlotStyle -> {RGBColor[1, 0, 0], RGBColor[0, 0, 1]}]