Coordinate Geometry: Circles in the Coordinate Plane

As a follow up to the previous lesson, we will now look at circles in the coordinate plane.

Transcript: Circles in the Coordinate Plane

In the previous video I suggested that a circle centered at the origin with a radius of r would have the formula x squared + y squared = r squared. We concluded this by thinking about distance in the x-y plane, and in particular thinking about the application of the Pythagorean theorem to the coordinate plane. Now we ask, what if the origin is not the center of the circle?

Equation

What is the equation then, if we have circle center somewhere else in the coordinate plane? Let’s think about this. We would have the circle centered in some general point. The center is H,K has the radius of R, we want to the equation of that circle. We’ll look at that little right triangle, the right triangle that has a hypotenuse of r.

It has a horizontal leg and a vertical leg. The horizontal leg is this distance right here between x and h. So would be x — h were is the absolute value of x minus h. Similarly, the vertical leg is this distance right here, between y and k. So y — k, or the absent value of that. Doesn’t matter because we’re gonna square those anyway.

And so the horizontal leg squared + the vertical leg squared must equal the high partner squared. Because the high partners is r and so that gives us the formula x minus h squared + y — k squared = r squared. And that is the general formula for a circle in the coordinate plane with center h, k and radius r.

Once again I will encourage you not simply to memorize that formula, but to understand the logic that underlies that formula.

Understanding the Formula for Circles in the Coordinate Plane Image by Aha-Soft

You will understand much more deeply if you understand where that formula comes from. If the radius = r and the center = h, k, then the equation of the circle is x — h squared + y — k squared = r squared.

Practice Problem

Okay, a square in the x-y plane has the vertices (-1, 1), (3, 1), (3, -3), and (-1, -3). A circle is tangent to all four sides. What is the equation of the circle? So they’re very nice to give us a diagram. We can see that, well, first of all, notice the radius of this circle is 2.

So right away, we know that the r squared side has to be 4. So all these ones that have 9 can’t be correct. We know the center is at +1, -1. The x-coordinate of the center is a positive number, the y-coordinate of the center is a negative number, as we can see from the diagram. The center is in the fourth quadrant.

And so this means that when we subtract, we’re gonna have x — 1. But the y, for the y we’re gonna have y- -1 to a minus- -1 is + 1, so it’s going to be x — 1 and y + 1. Those are going to be the two components of it. And so that has to be answer choice B.

Practice Problem

Here’s another question. A bit of an odd question. Pause the video and then we’ll talk about this.

So it gives us the equation of a circle, a square is drawn around the circle and each side is tangent to the circle. For each vertex of this square we compute the sum of the x and y coordinates the highest of the four sums is what?

It’s a very unusual question. So let’s think about this. First of all, from the equation, we know the center is -4,2, very important because it x 4. That means that the x coordinate of the center is -4, y — 2, that is the center at +2 and of course the radius is 5. Well, from the x coordinate of x = -4, if we go to the left five spaces, we get to x = -9.

We go to the right five spaces we get to x = +1. From the y coordinate of 2, we go down five spaces. We get to the line y = -3, and if we go up five spaces, we get to the line y = -7. So technically those four lines that’s two horizontal lines that those two vertical lines define the square. And so this means that the vertices would be the places where those lines intercept.

So the vertices are these four points. And we take the sum, we see that the highest sum is the one with the two positive numbers, 1 and 7, 1 + 7 = 8, so that’s the highest sum. And it kinda makes sense that the highest sum would be the one that would be on the upper-right side of the square, that the vertex on the upper-right side obviously is gonna have larger coordinates than any of the other three coordinates.

So the sum is 8. We go back to our choices and we choose 8.

Summary

In summary, if the radius is r and the center is (h, k) then the equation of the circle is x — h squared + y — k squared = r squared. Once again I would urge you do not simply memorize this by wrote, but understand the argument that produce this equation.

That will make you much more confident in using it.

Author

• Mike served as a GMAT Expert at Magoosh, helping create hundreds of lesson videos and practice questions to help guide GMAT students to success. He was also featured as "member of the month" for over two years at GMAT Club. Mike holds an A.B. in Physics (graduating magna cum laude) and an M.T.S. in Religions of the World, both from Harvard. Beyond standardized testing, Mike has over 20 years of both private and public high school teaching experience specializing in math and physics. In his free time, Mike likes smashing foosballs into orbit, and despite having no obvious cranial deficiency, he insists on rooting for the NY Mets. Learn more about the GMAT through Mike's Youtube video explanations and resources like What is a Good GMAT Score? and the GMAT Diagnostic Test.