I need to solve the equations:

First I try adding and subtracting the two equations to get:

But I can make no real progress from there.

Now the context of the question means that:

, and I am more than happy to make a few aproximations. I expect the binomial aproximation may be useful.

The required solution is:

I have posted the whole question on Physics help forum at:

http://www.physicshelpforum.com/phys...lleration.html