f(x) = -x + sqrt(x^(2) + 4)
a = 4.8 (slider from 0 to 100, step 0.1)
A: (a, f(a))
A': (-a, f(-a))
b = Tangent(A', f)
c = Tangent(A, f)
M = Intersect(b, c)
d: y > 10.8