# Common Core Standards: Math

#### The Standards

# High School: Geometry

### Expressing Geometric Properties with Equations HSG-GPE.A.1

**1. Derive the equation of a circle of given center and radius using the Pythagorean Theorem; complete the square to find the center and radius of a circle given by an equation.**

Ah, the circle. The basis of so many delicious baked goods: donuts, pies, cookies, and cakes. Always regular and similar to all of its circle friends. It tries so hard to fit in with the crowd, and yet we insist on labeling it, giving it an equation, shoving it into a 360 degree oven for 12 to 14 minutes, and trying our hardest to let it cool before cramming its deliciousness down our gullets.

Students should know that the equation for a circle is *x*^{2} + *y*^{2} = *r*^{2}. You're right, that does look eerily familiar. Sort of like the Pythagorean theorem (*a*^{2} + *b*^{2} = *c*^{2}.) Wouldn't it be great if *that* was all the students needed to know?

If only it were that simple. The equation for a circle is actually (*x* – *h*)^{2} + (*y* – *k*)^{2} = *r*^{2}, where (*h*, *k*) is the center of the circle. But when a circle is centered on the origin, or (0, 0), the equation simplifies to *x*^{2} + *y*^{2} = *r*^{2}. So actually, comparing it to the Pythagorean theorem comes in quite handy after all.

This circle is centered at (2, 3) with a radius 10, meaning that students have to adjust the left side of the standard circle equation. The adjustment should reflect how we would have to move the circle so that it is centered on the origin; in this case, two to the left and down three. Our equation ends up being (*x* – 2)^{2} + (*y* – 3)^{2} = *r*^{2}.

To find the radius, students can draw a right triangle *inside* the circle and use the Pythagorean theorem to find the length of the hypotenuse. It just so happens that the hypotenuse is also the radius of our circle. Since our hypotenuse here is 10, our equation is (*x* – 2)^{2} + (*y* – 3)^{2} = 100.

If students are having a hard time remembering whether to do add or subtract their *h*'s and *k*'s, they can simply think, "How would I move the center to the origin?" For example, if their circle was centered at (-5, -17), they'd have to adjust the left side by saying "plus 5" or "plus 17", since they'd have to move the center up and to the right, both of which are positive motions, or as we like to call them, good vibrations.

Perhaps your students find themselves in the lucky situation of having been given an equation and told to find the center and radius of the circle that equation describes. They will need to start by completing the square in the equation so that they can convert it into the standard form.

If they are having a hard time solving these kinds of problems, they might need a quick refresher on how to complete the square. Also, they should note that they're actually completing *two* squares, since both *x* and *y* are squared. For a standard about circles, we're sure using a lot of squares.