Question

Find the equation of the circle: circle with center (1,2) that passes through point (-2,-1)?

Answer 1

you know it looks like \((x-1)^2+(y-2)^2=r^2\) so all you need is \(r^2\) which is the square of the distance between \((1,2)\) and \((-2,-1)\)

Answer 2

use the distance formula without the annoying square root to find \(r^2\)