# The equation 3 x 2 + 3 y 2 + 6 x − y = 0 represents a circle and evaluate the center and radius of the circle.

### Precalculus: Mathematics for Calcu...

6th Edition
Stewart + 5 others
Publisher: Cengage Learning
ISBN: 9780840068071

### Precalculus: Mathematics for Calcu...

6th Edition
Stewart + 5 others
Publisher: Cengage Learning
ISBN: 9780840068071

#### Solutions

Chapter 1.8, Problem 108E
To determine

## To show: The equation 3x2+3y2+6x−y=0 represents a circle and evaluate the center and radius of the circle.

Expert Solution

The equation 3x2+3y2+6xy=0 represents a circle. The center of the circle is (1,16) and radius is 3736 units.

### Explanation of Solution

Given information:

The equation 3x2+3y2+6xy=0 .

Formula used:

In order to solve a quadratic equation, completing the square method is used that transforms the equation in the form of square trinomial.

Step1: Divide the equation by coefficient of x2 if it is not equal to 1.

Step 2: Take the square of the half of the coefficient of x and add it to both sides of the equation.

Step 3: Factor the equation.

The standard form of the equation of the circle is (xh)2+(yk)2=r2 , where (h,k) denote the center of the circle and r denote the radius.

Calculation:

Consider the equation 3x2+3y2+6xy=0 .

Recall that in order to solve a quadratic equation, completing the square method is used that transforms the equation in the form of square trinomial.

Step1: Divide the equation by coefficient of x2 if it is not equal to 1.

Step 2: Take the square of the half of the coefficient of x and add it to both sides of the equation.

Step 3: Factor the equation.

In the provided equation, divide the entire equation by 3,

x2+y2+2xy3=0

Add 1 to both the sides of the equation,

x2+y2+2xy3+1=0+1

Again add 136 to both the sides of the equation,

x2+y2+2xy3+1+136=0+1+136

Group the terms,

x2+y2+2xy3+1+136=0+1+136(x2+2x+1)+(y2y3+136)=3736

Factor out the trinomial, recall that (a+b)2=a2+2ab+b2 and (ab)2=a22ab+b2 .

Apply it,

x2+y2+2xy3+1+136=0+1+136(x2+2x+1)+(y2y3+136)=3736(x+1)2+(y16)2=3736

Recall that the standard form of the equation of the circle is (xh)2+(yk)2=r2 , where (h,k) denote the center of the circle and r denote the radius.

Convert the equation obtained above in standard form,

x2+y2+2xy3+1+136=0+1+136(x2+2x+1)+(y2y3+136)=3736(x+1)2+(y16)2=3736(x(1))2+(y16)2=(376)2

Compare, (xh)2+(yk)2=r2 and (x(1))2+(y16)2=(376)2 .

Here, h=1,k=16 and r=3736 .

Therefore, center of circle is (1,16) and radius is 3736 .

Thus, the equation 3x2+3y2+6xy=0 represents a circle. The center of the circle is (1,16) and radius is 3736 units.

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