parametriche cardioide

x = k (2 cos(t) - cos(2t))
y = k (2 sin(t) - sin(2t))

k = -0.8 (slider from -5 to 5, step 0.1)
a = CurveCartesian(k * (2 * cos(t) - cos(2 * t)), k * (2 * sin(t) - sin(2 * t)), t, 0, 6.28)