What is the equation for a circle?

Answer 1

The equation for a circle of radius r and centre (h, k) is:

(x - h)² + (y - k)² = r²

If the centre is the origin, the centre point is (0, 0), thus h = k = 0, and this becomes:

x² + y² = r²

Answer 2

x squared + y squared = r squaredFor a circle centered at (a, b), the equation is:

(x - a) squared + (y - b) squared = r squared