Circles Standard Equation (x - h)² + (y - k)² = r² Center: (h, k), Radius: r Key Formulas Circumference = 2πr | Area = πr² Arc length = (θ/360) × 2πr | Sector area = (θ/360) × πr² SAT Question Types Find center and radius from equation (may need to complete the square) Arc length and sector area Tangent lines are perpendicular to radius at point of tangency Central angle = arc measure Radians 180° = π radians | θ(rad) = θ(deg) × π/180