Power of a point w.r.t. a circle (Power = S₁ (value of S at point, for S=0 circle))
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For circles S₁=0, S₂=0, radical axis is S₁−S₂=0 (linear); locus of equal tangent lengths.
What each symbol means
| Symbol | What it stands for |
|---|---|
| S₁, S₂ | General circle equations |
When to use this
Coefficients of x² and y² equal (both circles normalized same) — usually both written with leading 1.