Question

What is the general form of the equation of a circle with a center at the (–4, 2) and a radius of 9?

I got (x + 4)^2 + (y – 2)^2 = 81

I just wanted to confirm whether it was correct or not. I'm pretty sure the signs change making it (-4,2) and the 81 is the diameter or circumference.

Answer 1

@amistre64

Answer 2

81 =r^2

Answer 3

(x-x1)^2+(y-y1)^2=r^2

Answer 4

@dricboylan

Answer 5

the general setup of a circle centered at (a,b) with radius r is:

(x-a)^2 + (y-b)^2 = r^2
(x+4)^2+ (y-2)^2 = 9^2

-a = 4
-b = -2

(a,b) = (-4,2) r^2 = 9^2 = 81

Answer 6

so thats fine

Answer 7

the "signs change" is a good thing to remember :)

Answer 8

So I was right?

Answer 9

you were right