# High School Math : Circle Functions

## Example Questions

### Example Question #1 : Center And Radius Of Circle Functions

Find the radius of the circle given by the equation:

Explanation:

To find the center or the radius of a circle, first put the equation in the standard form for a circle:  , where  is the radius and  is the center.

From our equation, we see that it has not yet been factored, so we must do that now. We can use the formula  .

, so .

and , so  and .

Therefore, .

Because the constant, in this case 4, was not in the original equation, we need to add it to both sides:

Now we do the same for :

We can now find :

### Example Question #1 : Understanding Circle Functions

Find the center of the circle given by the equation:

Explanation:

To find the center or the radius of a circle, first put the equation in standard form:  , where  is the radius and  is the center.

From our equation, we see that it has not yet been factored, so we must do that now. We can use the formula  .

, so .

and , so  and .

This gives .

Because the constant, in this case 9, was not in the original equation, we must add it to both sides:

Now we do the same for :

We can now find the center: (3, -9)

### Example Question #2 : Quadratic Functions

Find the -intercepts for the circle given by the equation:

Explanation:

To find the -intercepts (where the graph crosses the -axis), we must set . This gives us the equation:

Because the left side of the equation is squared, it will always give us a positive answer. Thus if we want to take the root of both sides, we must account for this by setting up two scenarios, one where the value inside of the parentheses is positive and one where it is negative. This gives us the equations:

and

We can then solve these two equations to obtain .

### Example Question #3 : Quadratic Functions

Find the -intercepts for the circle given by the equation:

Explanation:

To find the -intercepts (where the graph crosses the -axis), we must set . This gives us the equation:

Because the left side of the equation is squared, it will always give us a positive answer. Thus if we want to take the root of both sides, we must account for this by setting up two scenarios, one where the value inside of the parentheses is positive and one where it is negative. This gives us the equations:

and

We can then solve these two equations to obtain

### Example Question #1 : Circle Functions

Find the center and radius of the circle defined by the equation:

Explanation:

The equation of a circle is:  where  is the radius and  is the center.

In this problem, the equation is already in the format required to determine center and radius. To find the -coordinate of the center, we must find the value of  that makes  equal to 0, which is 3. We do the same to find the y-coordinate of the center and find that . To find the radius we take the square root of the constant on the right side of the equation which is 6.

### Example Question #1 : Finding The Center And Radius

Find the center and radius of the circle defined by the equation: