For circles, radius midpoints define smaller-circle {inversion circle} centers that intersect first circle at only one point and include larger-circle center. Radius diameters intersect first circles at points {inverse point}. Distance from first intersection to first-circle center times distance from inverse point to first-circle center equals r^2 {inversion constant}.
Circles {concentric circle} can have same center.
Circles {escribed circle} can touch three consecutive polygon sides, if two polygon sides extend.
Circles {great circle}| on spheres can have same radius as sphere.
Equation (x - a)^2 + (y - b)^2 + c^2 = 0 has radius = i*c {imaginary circle}.
Circle can touch three consecutive polygon sides {incircle} {inscribed circle}. Inscribed circle has center {incenter}.
Two circles {orthogonal circle} can intersect at right angles. Curves {orthogonal trajectory} can intersect all curve-family members at right angles.
Plane intersects sphere to make circle {small circle}. Small circle does not include great circle.
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Date Modified: 2022.0225