Find the equation of the circles?

Question

center at (1,2) passing through the point (-2,-1)

Answer 1

The general equation of a circle is

(x-h)² + (y-k)² = r²

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

The radial line from center to (-2,-1) is the hypotenuse of a right triangle with height 3 and base 3. By the Pythagorean theorem,

r² = (-3)² + (-3)²

= 18

The equation of the circle is

(x-1)² + (y-2)² = 18

http://www.flickr.com/photos/dwread/2797...

Answer 2

The radius of the circle has an absolute x component of 3 and a y component of 3.  So its value is 3sqrt(2).   The equation is (x-1)^2 + (y-2)^2 = 18