Formula of a Circle

Circle Equation

Review

The radius of a circle is any line that extends from the center of the circle to the edge.

Radius = half the length of the diameter

Diameter

The diameter of a circle is any straight line that extends from one edge of the circle through the center and to the opposite side. The value of the diameter is two times the radius.

The Standard Equation of a Circle Formula:

This is the standard equation formula of a circle. It is not as intimidating as it looks.

Let’s break it down:

-You only need to know two pieces of information to write the standard equation of a circle:

------The Center Point Coordinates (h,k) where h is the x-value and k is the y-value

-------The length of the radius r

Step 1: Find the center of the circle

X,Y cordinate

Find the center

Multiple Choice

What is the center of this circle?

• (-7, 1)

• (-7, -1)

• (7, 1)

• (7, -1)

Step 2: Find the radius of the circle

Find the radius

Step 3: Formula

Multiple Choice

Find the formula

• (x-7)² + (y-1)² = 9

• (x+7)² + (y-1)² = 9

• (x-7)² + (y+1)² = 9

• (x+7)² + (y+1)² = 9