(1).
Now, if then
.
Replacing in (1) we have .
If
Then: ..................(1)
and : ....................(2)
Now: .............I factored out from the first two terms and from the last two terms.
Now we can replace what's in brackets above with equations (1) and (2) respectively
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EDIT: Beaten by the great red_dog. Nice Solution, red_dog!