等比数列

Clear[a]

a[1] = 5;

a[n_] := 3*a[n - 1]

Clear[data1]

data1 = Table[a[k], {k, 10}]

p1 = ListPlot[data1, PlotStyle  ->  Hue[0.0]];

Clear[suma]

suma[n_] := Sum[a[k], {k, 1, n}]

Clear[data2]

data2 = Table[suma[k], {k, 10}]

p2 = ListPlot[data2, PlotStyle  ->  Hue[0.3]];

p3 = 
        Plot[5/3*3^x, {x, 0, 10}, 
          PlotStyle  ->  Hue[0.6]];

Show[p1, p2, p3];

<< Graphics`Graphics`

lp1 = 
        LogListPlot[data1, PlotStyle  ->  Hue[0.0]];

lp2 = 
        LogListPlot[data2, PlotStyle  ->  Hue[0.6]];

Show[lp1, lp2];

Table[suma[n + 1]/suma[n]  // N, {n, 1, 9}]

data2 = Table[suma[k], {k, 10}]

data2 - a[1]

data3 = (data2 - a[1])/3

Drop[data2, -1] == Drop[data3, 1]