Write the equation of the circle whose center is at the origin and whose diameter is 12.

The standard form of the equation of a circle is,

(x - h) ² + (y - k) ² = r²

where the center is at (h, k) and its radius is r. From the givens above, the equation of the circle is,

x² + y² = (12/2) ²

or equal to x² + y² = 36.