Learn math, science, English SAT & ACT from
highquaility study
videos by expert teachers
Thank you for watching the preview.
To unlock all 5,300 videos, start your free trial.
The Circle  Problem 3
Carl Horowitz
Carl Horowitz
University of Michigan
Runs his own tutoring company
Carl taught upperlevel math in several schools and currently runs his own tutoring company. He bets that no one can beat his love for intensive outdoor activities!
Start by writing the given equation of a circle in standard form: r^2 = (xh)^2 + (yk)^2. This may require completing the square. Gather the x variables and the y variables on one side of the equal sign, and the constant to the other side. Next, complete the square for both variables. Whatever is being added to one side when completing the square should also be added to the other side to keep the equation balanced. Once the equation is in standard form, you can determine the coordinates of the center and the radius by look at the h, k, and r values. To find the coordinate of the center, extract the h and k values. Since the quantities are (xh) and (yk), that means that the value of just "h" and "k" will have the opposite sign. In other words, if the quantity is (x1), then h = 1. If the quantity is (x+3), then h = 3. Since r is being squared, the radius is the square root of the constant value. In other words, if r^2= 25, the radius is 5.
So we're good at finding the center and the radius for a circle when we have it in x² plus y² is equal to r² form, but we're not always going to get equations in that form. So like what I have behind me is just this string of information. What we want to do is get it into the form where we can recognize what we're looking at and the way to do that is by completing the square, our favorite.
So what we want to do is isolate our x's together, isolate our y's together and get the constant term to the other side by itself.
So just doing a little bit of rearranging, what I'm going to do is x² minus 2x leave a little space to complete the square there, plus y² plus 6y leave a little space after that and now this is equal to 15. So now we just want to complete the square, x remember we divide our middle term by 2 so 2 divided by 2 is 1 that is what goes here, 1² that is what ends up going over here. So we added one into this parenthesis, so to keep it balanced, we have to add one over to the other side as well.
Doing the same thing with the y's plus again divide our middle term by 2, so 6 divided by 2 is 3 and that ends up being squared, so we actually end up adding 9 keeping the sides balanced again we have to add 9 to the other side, and so what we end up with on the right is 15 plus 1, 16 plus 9, 25.
By completing the square, we were able to get our equation just from a string of information into a form that we recognize, so now all we have to do is extract the information we need. We're asked for the center, so x² minus the center is 1, y minus the center that will be 3 so our center is 1,3 and that's equal to our radius 25 square root of that is 5 so the radius is equal to 5.
So finding the center and radius of a circle by completing the square.
Please enter your name.
Are you sure you want to delete this comment?
Carl Horowitz
B.S. in Mathematics University of Michigan
He knows how to make difficult math concepts easy for everyone to understand. He speaks at a steady pace and his stepbystep explanations are easy to follow.
Concept (1)
Sample Problems (11)
Need help with a problem?
Watch expert teachers solve similar problems.

The Circle
Problem 1 13,993 viewsEquation for a circle centered at (1,2) with radius 5.

The Circle
Problem 2 11,110 viewsGive the center and radius of the circle defined by
2(x − 4)² + 2(y − 1)² = 72 
The Circle
Problem 3 10,601 viewsGive the center and radius of the circle defined by
x² + y² + 6y − 2x − 15 = 0 
The Circle
Problem 4 10,492 viewsFind the equation of a circle that has a diameter with endpoints at (3,1) and (5,5).

The Circle
Problem 5 2,316 views 
The Circle
Problem 6 2,149 views 
The Circle
Problem 7 2,074 views 
The Circle
Problem 8 2,022 views 
The Circle
Problem 9 2,046 views 
The Circle
Problem 10 2,076 views 
The Circle
Problem 11 2,016 views
Comments (0)
Please Sign in or Sign up to add your comment.
·
Delete