Hi Nio,
Use exact values
intsom[f_, n_, a_, b_] :=
Module[{t = 0, fen = {}},
While[t < n + 1, c = f[a + ((b - a)/n)*t]; fen = Append[fen, c]; t++];
T = ((b - a)/n)*(Total[fen] - 1/2*(f[a] + f[b]))]
list1 = {};
list2 = {};
Do[list1 =
Append[list1,
Abs[39 - intsom[g, 20*n, 2, 5] - (intsom[g, 20*n, 2, 5] - intsom[g, 10*n, 2, 5])/3]], {n, 10, 100}]
ListPlot[list1]
