colorfull geometry

[BSOB]

see avatar for image.
ok, i have two circles (black) on a plain (white) of which i know their center points (blue) in x,y and their radii (green). i need to find the 4 points (yellow) at which two lines (red) would be tangent to both cirles.

...help

[BSOB]

this might not look like a visual basic issue... but it is going into my vb app, and if i post on a math forum, ill get a bunch of nothing. i appreciate your patients with my posts.