r(x) = sqrt(x)
b = 7.7 (slider from 0 to 20, step 0.1)
a = CurveCartesian((r(t) * cos(t)), (r(t) * sin(t)), t, 0, b)
Text1: "r(x)=" + (LaTeX[r]) + ""
u = Vector(((r(b) * cos(b)), (r(b) * sin(b))))
α = Angle(u)