f(x) =

f(x) = If[0 ≤ x < 2, (1 / 2 * x) + 1, If[2 ≤ x < 3, 2, If[3 ≤ x < 9, (-(x - 3)) + 2, If[9 ≤ x < 20, 0]]]]
Text4 = Text("G. Roolfs")
a = 0.585 (slider from 0 to 9, step 0.001)
A = Integral(f(x), 0, a)
Eingabefeld1 = Textfield(f)
B: (a, A)