a = 3 (slider from -3 to 3, step 0.1)
f(x) = 2.0^(-(x - 0.20134697674418595)) + 0.2996297674418602
A: (a, f(a))
a_1 = Tangent(A, f)
B = Intersect(a_1, xAxis)
C: B + (3.0, 0.0)
α = Angle(C, B, A)