9
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Equation of a Circle

Question 9 • EAP-Geometry B CR

Consider a circle whose equation is x2 + y2 + 4x – 6y – 36 = 0. Which statements are true? Check all that apply.

Answer
A
To begin converting the equation to standard form, subtract 36 from both sides.
B
To complete the square for the x terms, add 4 to both sides.
C
The center of the circle is at (–2, 3).
D
The center of the circle is at (4, –6).
E
The radius of the circle is 6 units.
F
The radius of the circle is 49 units.

Explanation

Move the constant to the right: x² + 4x + y² − 6y = 36. Completing the square for x² + 4x requires adding 4 to both sides, so B is true; the completed-square form is (x + 2)² + (y − 3)² = 49. The center is (−2, 3), making C true, and the radius is √49 = 7—not 6 or 49. A is false because you add 36 to both sides to move the constant.

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