@Koro If we consider a 3 by 3 matrix B, whose ith row is ($l_i,m_i, n_i$).
Then note that $BB^T =I$.
So $B^TB=I$ (because $B^T$ is the inverse of B).
If we look at the entries of $B^TB$ and compare them with the entires of I. This totally gives me the result. This is what you meant, right? Thank you soooooooo much!!!!!! **I do get it now!, And I was able to understand it completely, thanks to you** I am facing a deficit of words to thank you... 😊 I remain indebted to you.