consider
(1)
In the r2
integral measure theta from r1 so that theta 12 in the picture is the angle
theta in the integral.
(2)
let μ equal
cos θ
(3)
In the mu
integration let
(4)
so that
(5)
Allowing for the
fact that r1 can be both larger and smaller than r2, this
becomes
(6)