Length of tangent from a point to a circle (ℓ = √S₁ (S=x²+y²+2gx+2fy+c, S₁=substitute point))
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ℓ = √S₁ (S=x²+y²+2gx+2fy+c, S₁=substitute point)
What each symbol means
| Symbol | What it stands for |
|---|---|
| S | Circle expression =0 |
| S_1 | Value after replacing x²→xx₁ etc at point |
When to use this
Point outside ⇒ S₁>0; on circle S₁=0.