At http://mathworld.wolfram.com/Circle.html, is the parameterized form of y incorrect?
there, It states y = (2t)/(1+t^2), where t is radians from 0 to 2*Pi.
You can see right off, y will never go negative but should be between +1 and -1.
The x value plots ok.
If you plot the values with radius 100, you can see it as well.
Plot[100(2t/(1 + tt)), {t, 0, 2Pi}]; Plot[100((1 - t^2)/(1 + t^2)), {t, 0, 2Pi}]
