Calculate fillet between arc and a line

I am trying to find some algorithms or formulas for doing a fillet or blend operation on the intersection of an arc and a line segment given a particular radius. In other words, I need to find the equation for the circle that will have tangent points on both the line and the arc.

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Eventually, this has to be calculated using VB.Net so any sample code will be appreciated.

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as a geometry problem:

1. calculate the line that is parallel to the initial line and r distance away
2. calculate the arc that is parallel to the original arc and r distance away
3. find the intersection of those two items

easier if the two are lines in the first place...