Question 581859
A circle with radius r, centered at point (h,k) has the equation
{{{(x-h)^2+(y-k)^2=r^2}}}
Your circles, centered at (0,0) will have the equation
{{{x^2+y^2=r^2}}}
For a radius of 2, since {{{2^2=4}}}, the equation would be
{{{x^2+y^2=4}}}
Solving for y, you will get two functions, representing the top half of the circle, and the bottom half of the circle.
{{{x^2+y^2=4}}} --> {{{y^2=4-x^2}}}
Then for the top half
{{{y=sqrt(4-x^2)}}}
and for the bottom half
{{{y=-sqrt(4-x^2)}}}