We are assuming that , right? (If not, you could have, say, and and then would be negative.)
Looking at the expression, the best thing to do first looks like eliminating :
We also have , i.e. . So
If prove that
is equal to
It seems like a mess. Any suggestions on where to start?
Btw, this problem is from Higher Algebra by H.S Hall and S. R Knight, a textbook that is still used in India even though it was written in the 1800's.