Answer: x² + y² = r².
- A x + y = r
- B x² + y² = r
- C x² + y² = r²
- D x² - y² = r²
Correct answer: C. x² + y² = r²
Explanation: Standard equation of circle at origin: x² + y² = r².
The slope of a line is the ratio of vertical change (rise) to horizontal change (run) between any two points on it - equivalently, the tangent of the angle θ the line makes with the positive x-axis.