Find the center and radius of the circle with equation x2 + y2 − 4x + 16y + 4 = 0
Answer:

Get the constant (4) on the other side of the equation.
x^2 + y^2  4x + 16y = 4 
Arrange the terms into the format for an equation of a circle.
(x  h)^2 + (y  k)^2 = r^2
2a. Match up like terms
x^2  4x + y^2 +16y = 4
2b. Complete the square
(x^2  4x + 4) + (y^2 + 16y + 64) = 4 + 4 + 64
2c. Factor and simplify
(x  2)^2 + (y + 8)^2 = 8^2
Now it’s in the format for an equation of a circle and you can find your radius and the center.