What is the equation of a circle centered at the origin with radius 2?

The equation of a circle centered at the origin (0, 0) with a radius of 2 is given by the formula (x^2 + y^2 = r^2), where (r) is the radius. Substituting the radius into the equation, we get (x^2 + y^2 = 2^2), which simplifies to (x^2 + y^2 = 4).