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I need to calculate the two tangent points of a circle with the radius $r$ and two lines given by three points $Q(x_0,y_0)$, $P(x_1,y_1)$ and $R(x_2,y_2)$.

Sketch would explain the problem more. I need to find the tangent points $A(x_a,y_a)$ and $B(x_b,y_b)$. Note that the center of the circle is not given. Please help.

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    Writing $C$ for the center of the circle and $\theta$ for the angle at $P$, note that $CB/PB = \tan(\theta/2)$. Since $CB = r$, we have $PB = r \cot(\theta/2)$. Does that get you started?2012-02-14
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    @DayLateDon Well, yes. Then how to find the θ?2012-02-14
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    For $\theta$, see here: http://planetmath.org/encyclopedia/AngleBetweenTwoLines.html2012-02-14

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