In particular, a tangent line is perpendicular to the radius that goes through the point of tangency. It is called the power of P with respect to the circle. The locus of points from which the segment AB subtends a fixed angle is an arc of circle. Let P , P , P , P be points in the plane, and let , for 1 i , j 4, be the distance between and . This is equivalent to Ptolemy's formula for cyclic quadrilaterals .
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