Center and radius of x^2 + y^2 - 6x = 7

Step by step

The question

Question: find the center and the radius of x² + y² − 6x = 7.

1 Group x and y

Put the x terms together in a bracket, and the y terms together.

2 Complete the square for x

To make x² − 6x a full square: half of −6 is −3.

(−3) times (−3) is 9.

So we add 9.

We do it on both sides, so they stay equal.

Careful!

What we add on the left, we add on the right too.

Then the two sides stay equal.

3 Write as squares

Each bracket is now a full square.

On the right, 7 + 9 is 16.

So (x − 3)² + y² = 16.

x² − 6x + 9 is (x − 3)².

4 Read the center and radius

Compare with the circle rule: (x − h)² + (y − k)² = r².

The signs turn: x − 3 gives h = 3.

y² gives k = 0.

So the center is (3, 0).

r² is 16.

So r is the square root of 16: 4, because 4 times 4 is 16.

So the center is (3, 0) and the radius is 4.

5 Check

Let's check with a point on the circle: (7, 0), 4 to the right from the center.

Put it in the first equation.

We get 7, the same as the right side.

So the center and the radius are right!

Let's look back.

First: group x and y.

Then: complete the square for x.

Then: write as squares.

Then: read the center and radius.

Last: check.