Answer
$d = \frac{|Ax_0 + By_0 + C|}{\sqrt{A^2 + B^2}}$.
Explanation
This formula is derived from vector projection and is a standard tool in CSAT geometry problems involving tangent lines to circles.
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What is the formula for the distance $d$ from a point $(x_0, y_0)$ to the line $Ax + By + C = 0$?
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