Power of a point (Power of P w.r.t. circle S=0 is S₁ = value of S at P; equals CP²−r²)
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Power of P w.r.t. circle S=0 is S₁ = value of S at P; equals CP²−r²; external: positive and equals tangent-length squared.
What each symbol means
| Symbol | What it stands for |
|---|---|
| C | Center |
| r | Radius |
| S₁ | Algebraic power |
When to use this
Consistent sign: inside negative, on zero, outside positive (standard convention for S=x²+y²+2gx+2fy+c with circle written that way).