What is the instantaneous radius of curvature?

An object is moving along an elliptical track and its x and y coordinates are recorded in meters by a stroboscope as follows:

x = ct

y = 1 - 2 c2^ t^2

with c=0.707 and t measured in seconds. What is the instantaneous radius of curvature at t = T?

The diagram below is the trajectory of the object recorded by a stroboscope flashing at 5 Hz.

The help notes for Q2.22 may be helpful. You were given an expression for a(t) there and r(t) here. Otherwise, the same approach should apply.

T[s] = 0.32;

http://micko.dyndns.org/elipse.jpg