Pair of tangents from an external point (SS₁=T² gives combined equation of two tangents from (x₁,y₁))
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Through common points of circles S₁=0 and S₂=0: family S₁+λS₂=0 (λ≠−1); λ=−1 gives radical axis.
What each symbol means
| Symbol | What it stands for |
|---|---|
| λ | Parameter |
| S₁, S₂ | Circle equations with same x²+y² coefficient |
When to use this
Circles must be written with matched x²+y² coefficients.