圆A,圆B都相外切于P,Q,AP平行BQ,
过B做AP垂线,垂足M,AM=R-r,AB=R+r
AB于l所成的锐角为∠MBA=a
sina=(R-r)/(R+r)=3/7
a=arcsin(3/7)
直线AB于l所成的锐角arcsin(3/7)