High School Math : Understanding Circle Functions
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Example Questions
Example Question #1 : Center And Radius Of Circle Functions
Find the radius of the circle given by the equation:
To find the center or the radius of a circle, first put the equation in the standard form for a circle: 
From our equation, we see that it has not yet been factored, so we must do that now. We can use the formula 
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 #2 : Circle Functions
Find the center of the circle given by the equation:
To find the center or the radius of a circle, first put the equation in standard form: 
From our equation, we see that it has not yet been factored, so we must do that now. We can use the formula 
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)
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