$ f_a(x)\,=\,a(ax+1)\hspace{0.2ex} e^{-ax} $

a = 1.7 (slider from 0 to 10, step 0.1)
f(x) = a * (a * x + 1) * exp(-a * x)
f'(x) = f'(x)
f''(x) = f''(x)
A = Intersect(f'', xAxis, (0.5882352941176482, 0))
b = Line(A, yAxis)
B = Intersect(f, b, (0.5882352941176482, 1.2507900999829025))
c = Tangent(B, f)
Text1 = LaTeX(f, true, true)
C = Intersect(f, xAxis, (-0.5882352941176471, 0))
D = Intersect(c, xAxis)
E = Intersect(f, f', (-0.2178649237472767, 1.5501862724493933))
Vieleck1, d, b_1, c_1 = Polygon(B, C, D)