How do you find the equation of a circle that passes through points (-8,-2)(1-,-11) and (-5,9)?

1 Answer
Jun 1, 2018

The equn. of a circle is :

7x^2+7y^2-52x-10y-912=0

Explanation:

Let the equation of the circle be

x^2+y^2+2gx+2fy+c=0,

Since the circle passes through (-8,-2), (1,-11) and (-5,9)

(-8)^2+(-2)^2-16g-4f+c=0
1^2+(-11)^2+2g-22f+c=0
(-5)^2+9^2-10g+18f+c=0

These equations simplify to

-16g-4f+c+68=0 -----------(1)
2g-22f+c+122=0 -----------(2)
-10g+18f+c+106=0 ----------(3)

Subtracting (1) from (2)
18g-18f+54=0 ------------(4)

Subtracting (1) from (3)
6g+22f+38=0 -------------(5)

Multiply (5) by 3
18g+66f+114=0 ------------(6)

Subtracting (4) from (6)
84f+60=0

therefore f= -5/7

Substituting the value of f into (4)

18g+90/7+54=0
18g+468/7=0
18g=-468/7

Divide both sides by 18
g=-26/7

Substituting the value of f and g into (1)
-16(-26/7)-4(-5/7)+c+68=0
416/7+20/7+c+68=0
c+912/7=0
c=-912/7

Hence, the equation of the circle is

x^2+y^2+2(-26/7)x+2(-5/7)y-912/7=0
x^2+y^2-52/7x-10/7y-912/7=0

Multiply through by 7
therefore 7x^2+7y^2-52x-10y-912=0

In the form
(x-a)^2+(y-b)^2=r^2

(x-26/7)^2+(y-5/7)^2=7085/49