# show that is a circleshow that something is the equation of a circle x² − 4x + y² − 2y = 11

*print*Print*list*Cite

### 2 Answers

An expression that can be represented in the form (x - a)^2 + (y -b)^2 = r^2 is the equation of a circle with center at (a, b) and radius r. It has to be of the form given and not just one that has x^2 and y^2 terms as even the equation of an ellipse has x^2 and y^2.

The given equation x^2 − 4x + y^2 − 2y = 11

=> x^2 - 4x + 4 + y^2 - 2y + 1 = 11 + 4 + 1

=> (x - 2)^2 +( y - 1)^ = 4^2

This is a circle with center (2,1) and radius 4

To prove that the given equation is the equation of a circle, we must recognize this form,

(x − a)^2 + (y − b)^2 = r^2.

a and b are the coordinates of the center of circle and r is the radius.

Therefore, we'll create perfect squares in both *x* and *y*.

To complete the square in *x*, we'll add 4 both sides.

To complete the square in *y*, we'll add 1 both sides.

This is the equation of a circle of radius 4, whose center is at (2, 1).