Trig
posted by HELP .
Please help. I can't get this problem at all.
When two bubbles cling together in midair their common surface is part of a sphere whose center D lies on the line passing through the centers of the bubble. Also, angles ACB and ACD are 60 degrees.
Show that the radius r of the common face is given by r=ab/ab.

sure you don't mean the common surface is a circle?
go to wolframalpha and enter "double bubble" to see a good writeup.