A
circle is the set of points in a plane that are equidistant from one point. That one point is called the
center of the circle, and the distance from it to any point on the circle is called the
radius of the circle. The standard form for the equation of a circle with its center at (0, 0) and with a radius of length
r is represented by the equation
Example 1: Graph
Recognize that
is the equation of a circle centered at (0, 0) with
, as shown in Figure
1 .
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Figure 1
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Circle in standard position.
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Example 2: Find the standard form for the equation of the circle centered (0, 0) with a radius of
The standard form for a circle centered at (0, 0) with a radius of
r is
Replacing
r with
, the equation becomes
Therefore,
is the standard form of the equation of a circle centered at (0, 0) with a radius of
The standard form for a circle centered at (
h,
k) with a radius of
r is
Example 3: Graph the equation
.
This equation represents a circle centered at (3, –2) with a radius of
, as shown in Figure
2 .
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Figure 2
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Circle offset down and right.
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Example 4: Find the standard form for the equation of the circle centered at (–6, 2) with a radius of
.
The standard form for the equation of a circle centered at (
h,
k) with radius
r is
Replacing
h with –6,
k with 2, and
r with
, the equation becomes
Therefore,
, which is the standard form of the equation of the circle centered at (–6, 2) with radius of
Example 5: From the equation given, find the center and radius for the following circle. Then graph the circle.
This equation can be rewritten as
Now, complete the square for each variable and add that amount to each side of the equation.
This circle is centered at (4, –6) with a radius of 8, as shown in Figure
3 .
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Figure 3
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The graph of Example 5.
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Linear Sentences in One Variable
Conic Sections
