The standard circle equation is (x – h)^{2} + (y – k)^{2} = r^{2}, with the coordinate of the center being (h,k) and the radius being r.

A problem might be,

A circle has a radius of √5 units and is centered at (2.1,-5.4). What’s the equation of the circle?

We can solve this using the circle equation.

(x – h)^{2} + (y – k)^{2} = r^{2}, and h is the x-coordinate of the center which is 2.1, k is the y-coordinate of the center which is -5.4, and r is the radius of the circle which is √5. We can rewrite the equation as (x – 2.1)^{2} + (y – (-5.4))^{2} = √5^{2}, or (x – 2.1)^{2} + (y + 5.4)^{2} = 5.