Answer: x² + y² = r².
- A x² + y² = r²
- B x² − y² = r²
- C x + y = r
- D xy = r²
Correct answer: A. x² + y² = r²
Explanation: Every point at distance r from the origin satisfies x² + y² = r².
All four conics form a single family distinguished only by eccentricity: a circle is the most "closed" (e=0), an ellipse is an elongated closed curve, a parabola is the borderline open curve (e=1), and a hyperbola has two separate open branches (e>1).
Concept context
Study circles, parabolas, ellipses, and hyperbolas as curves formed by intersecting a plane with a double cone, with their standard equations and key properties.