Geometry and TrigonometryEasyEquation of a circle centred at the origin
Geometry and Trigonometry practice question
Which of the following is an equation of the circle in the xy-plane with centre (0, 0) and radius 3?
- A. x² + y² = 9Correct
- B. x² + y² = 3
- C. x + y = 9
- D. (x − 3)² + y² = 9
Answer: A. x² + y² = 9
A circle centred at the origin with radius r has equation x² + y² = r², and r² = 9.
Why the other answers are wrong
- B. x² + y² = 3
- The right side must be r², not r.
- C. x + y = 9
- This is a line.
- D. (x − 3)² + y² = 9
- This circle is centred at (3, 0).
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