The center of a circle is at (-3, 1) and it’s radius is 9. what is the equation of the circle? (x - 3)^2 + (y + 1)^2 =81 (x - 3)^2 + (y + 1)^2 = 18 (x + 3)^2 + (y - 1)^2 =81 (x + 3)^2 + (y - 1)^2 =18

(x + 3)² + ( y - 1)² = 81

Step-by-step explanation:

To go in reverse, you need to remember that in the equation:

(x - a)² + (y - b)² = r²

a is the x coordinate of the centre and b is the y coordinate of the centre.

As -3 is the x coordinate, we can replace a with 3, and then replace  b with 1:

(x - -3)² + (y - 1)² = r²

The two negatives change to a positive:

(x + 3)² + ( y - 1)² = r²

If r is already given as 9, r² is simply 9²

(x + 3)² + ( y - 1)² = 9²

(x + 3)² + ( y - 1)² = 81

So the equation of the circle is (x + 3)² + ( y - 1)² = 81