Which variable is commonly used to denote the radius in circle formulas?

• A. d
• B. s
• C. r
• D. θ

Answer: (C)

Radius is the distance from the center of a circle to any point on its edge, and the symbol r is the standard way to denote that distance in circle formulas. This is why you see it in the key relationships: area equals π r^2, circumference equals 2π r, and the circle equation (x − h)^2 + (y − k)^2 = r^2. The diameter, which is the full width across the circle, is 2r, and d is sometimes used for diameter rather than radius. Other symbols like s (often arc length) or θ (an angle) don’t represent the radius. So r is the conventional and most consistent choice for the radius.