you can start to reason it through a little also...
We can write out the definitions for the terms on the right hand side...
Now we see we need to have under the line on the left hand side,
therefore as we can only multiply by 1 (so the ratio stays the same),
we multiply the left hand side by
as this gets us that term in the denominator,
however we see later on, that this term comes in anyway, so it ends up being unnecessary!
Now we are not supposed to have under the line
so we need to fish that out!
We therefore multiply by 1 again, this time using the "conjugate" of