Test:
a) Let the slider value h move towards zero.
b) Change point A and try again.
c) Change function f(x) and restart.

h = 1.215 (slider from -2 to 2, step 0.005)
f(x) = x^(2) - (4 * x) + 5
A = Point(f)
a = Tangent(A, f)
deltay_1: f(x(A) + h) - y(A)
B: (x(A) + h, f(x(A) + h))
b = Line(A, B)
C: (x(A), 0)
D: (x(B), 0)
hx = Segment(C, D)
E: (0, y(A))
F: (0, y(B))
deltay = Segment(F, E)
tekst1: "With the function: f(x) =  " + f + "
and pointt A = " + A + "  ie.  x0 = " + (x(A)) + " and h = " + h + " 
The point B = " + B + "

1. step:    
Δ y = f(x0+h) - f(x0) =
Δ y = " + (f(x(A) + h)) + " - " + (y(A)) + " = " + deltay_1 + "

2. step:
Δ y / h  = (f(x0+h) - f(x0)) / h = 
Δ y / h  =  " + deltay_1 + " / " + h + " = " + (deltay_1 / h) + "
and the blue line is:  " + b + "

3. step:
The limit is:  " + (Slope[a]) + "
and the tangent line is:  " + a + ""