A little Metapost problem.

[David Arnold <darnold@northcoast.com>]

Hans, et al,

Attached is a little doc. At the bottom of the page, I've attempted to draw
to circles tangent to one another. The line PQ is supposed to be a common
tangent line, that is, it is supposed to be tangent to both circles. 

I used the formula to compute theta, but the asin routine in Metapost is
not that accurate and the line dips a little too low on the first circle,
and a little high on the second circle.

I'd love to see some Metapost experts demo some cute ways of calculating
the common tangent PQ. Eventually, when I provide answers for my students,
I will need to know that points A and B where the common tangent touches
the two circles, as I will need to draw in radii of the circles
perpendicular to the common tangent to explain how to do the problem.