Sol[n_, a_] := 
      Module[{D = {{0, 1}}, s = 1}, 
        Do[s = (1 - (2/n))^k // N; 
D = 
            Append[D, {N[k/n], s}], 
{k, 1, a*n}]; 
        D]

Sol[3, 2]

(1 - (2/3))^2 // N

ListPlot[Sol[20, 2], PlotJoined -> True]

Plot[Exp[-2 x], {x, 0, 2}]