Geometry and TrigonometryMediumRadius from the equation of a circle
Geometry and Trigonometry practice question
In the xy-plane, the graph of x² + y² = 36 is a circle. What is the radius of the circle?
- A. 6Correct
- B. 36
- C. 18
- D. 3
Answer: A. 6
The equation x² + y² = r² describes a circle centred at the origin with radius r. Here r² = 36, so r = √36 = 6.
Why the other answers are wrong
- B. 36
- 36 is r², the radius SQUARED. Take the square root to get the radius.
- C. 18
- 18 is half of 36. The radius is the square root of 36, not half of it.
- D. 3
- 3 would give r² = 9, and the equation would read x² + y² = 9.
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