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6:50 AM
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Q: Proving $(ac+bd)^2+(ad-bc)^2=(a^2+b^2)(c^2+d^2)$ with various solutions.

RDK $(ac+bd)^2+(ad-bc)^2=(a^2+b^2)(c^2+d^2)$ Solutions in the answers. P.S. Please add more solutions in the answer line if you have any.

For anybody interested, this is one of two parts of the Brahmagupta–Fibonacci identity. — John Omielan 59 mins ago
I guess would actually be suitable here. But the main question is whether the newly created tag should be kept.
 

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