Roman C. answered 04/04/13
All we need to do is make use of the identity
sin2 t + cos2 t = 1
To do this we will square both parametric equations to get
x2 = 9 sin2 t and y2 = 4 cos2 t
x2/9 = sin2 t and y2/4 = cos2 t
Now wee add the two equations
x2/9 + y2/4 = sin2 t + cos2 t
(x/3)2 + (y/2)2 = 1