Question 940878
The standard form equation for a circle is:

{{{(x-h)^2+(y-k)^2=r^2}}}

In this equation the center is at (h,k) and r is the radius
For example:
{{{(x-3)^2+(y+4)^2=16}}}
is a circle with center at (3,-4) and a radius of 4

In the equation:
{{{x^2 + y^2 = 4}}}
h and k are 0, so the center is at the origin, (0,0) and the radius squared is 4 so the radius is 2
Graph:
{{{ graph( 500, 500, -5, 5, -5, 5, sqrt(4-(x^2)),-sqrt(4-(x^2))) }}}