r_1(x) = 2 / cos(x)
r_2(x) = (-2 * tan(x))
r(x) = r_1(x) + r_2(x)
θ = 9.7 (slider from -5 to 10, step 0.1)
P: (r(θ); θ)
K: (θ, r(θ))
f = CurveCartesian((r(α); α), α, 0, (2 * pi))