Is parameterized circle incorrect at mathworld.wolfram.com/Circle.html

Questions
,
1

At Circle -- from Wolfram MathWorld, 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, 2*Pi}]

2

Try this:

ParametricPlot[{(1 - t^2)/(1 + t^2), (2 t)/(1 + t^2)}, {t, -2, 2}, 
 AspectRatio -> 1]