δ

f(x) = sin(x)
A = Point(f)
ε = 1.4 (slider from 0 to 5, step 0.1)
B: (0, y(A) - ε)
C: (0, y(A) + ε)
δ: ε
inputBox1 = Textfield(δ)
D: (x(A) - δ, 0)
E: (x(A) + δ, 0)
e = OrthogonalLine(D, xAxis)
g = OrthogonalLine(E, xAxis)
F = Intersect(f, e, (1.125266398933067, 0.9023824270455354))
G = Intersect(f, g, (3.9252663989330667, -0.7058863827588189))
j = Segment(D, F)
k = Segment(E, G)
a = OrthogonalLine(F, yAxis)
b = OrthogonalLine(G, yAxis)
H = Intersect(yAxis, a)
I = Intersect(b, yAxis)
c = Segment(H, I)
d = Segment(C, B)