We are all familiar with Fibonacci-Brahmagupta’s identity:
I am trying to find whether there is a similar identity:
$$(a^2-mb^2)(c^2-nd^2)=x^2-py^2$$ where $p,m,n$ are not all equal.
If this problem has already been solved, please save me the trouble by pointing me at a reference. Otherwise, any hint will be greatly appreciated.Thanks.