Question

What is the equation of a circle centered at (1,-4) and a diameter 18?

Answer 1

(x-1)^2 + (y+4)^2 = 81

Explanation:

The equation of a circle can be written as

(x-h)^2 + (y-k)^2 = r^2

where ( h,k) is the center and r is the radius

The center is ( 1,-4)

and the radius is d/2 = 18/2 = 9

(x-1)^2 + (y- -4)^2 = 9^2

(x-1)^2 + (y+4)^2 = 81

Answer 2

(x - 1)² + (y + 4)² = 81

Explanation:

Circle Formula: (x - h)² + (y - k)² = r²

(h, k) is the center

2r = d

Step 1: Find r

18 = 2r

r = 9

Step 2: Plug known variables into formula

(x - 1)² + (y + 4)² = 9²

Step 3: Evaluate

(x - 1)² + (y + 4)² = 81