Circle P has a radius of 8 units with center P at (2, -1). Which equation defines circle P?

(x – 2)2 + (y + 1)2 = 64
(x + 2)2 + (y – 1)2 = 64
(x – 2)2 + (y + 1)2 = 8
(x + 2)2 + (y – 1)2 = 8

Question

Circle P has a radius of 8 units with center P at (2, -1). Which equation defines circle P?

(x – 2)2 + (y + 1)2 = 64
(x + 2)2 + (y – 1)2 = 64
(x – 2)2 + (y + 1)2 = 8
(x + 2)2 + (y – 1)2 = 8

callisto · Answer

centre - radius form :
(x – h)^2 + (y + k)^2 = r^2
where (h,k) is the centre of circle r is the radius
can you try it now?

anonymous · Answer

(x-2)^2 + (y+-1)^2=r^2
4+2=12?

callisto · Answer

sorry 
centre - radius form :
(x – h)^2 + (y - k)^2 = r^2

callisto · Answer

(x-2)^2 <- correct

callisto · Answer

r = radius , that is given in the question..

callisto · Answer

can you try again using the right one ?

anonymous · Answer

4+(y-k)^2=r^2
4+2=6 = 16
=96?

callisto · Answer

nope...

callisto · Answer

you just need to put h = 2, k =-1 and r=8 into the equation (x – h)^2 + (y - k)^2 = r^2
can you try again?

anonymous · Answer

(x-2)+(y--1)=8^2=

Am I supposed to multiply them by 2 because that's what I'm doing and it keeps coming out wrong.

callisto · Answer

hmm.. 8^2 = 8*8 = 64

callisto · Answer

$$8^2 \neq 8\times2$$